These are all of the known color varieties that are currently known for each species of dragon from the How to Train Your Dragon franchise. Most dragons have color varieties, and most appear to change their color when they are Titans (according to the Dragons: Rise of Berk game). There is one example of this in Total Nightmare.
Armorwing
Grey
Purple
Greysh tan
Orange
Dark purple and green
Green and yellow
Green (Titan Wing)
Bewilderbeast
White and charcoal
Dark gray and dark red
Boneknapper
Pale Green with White bones
Purple with white bones
Dark red and orange with white bones
Greenish grey with dark red highlights
Orange and pink with white bones (Titan)
Buffalord
Brown and bronze
Green with yellow stripes and brown spots
Orange, gold and black
Bronze and green
Shades of pink and white with dark spots (Titan Wing)
Bronze, red, and white
Catastrophic Quaken
Brownish black with whitish tan stripes and spikes
Light brown with tan spikes
Purple with light brown
Dark reddish brown and glowing red
Purple with tan spikes (Titan)
Changewing
Red
Green
Light green and pink
Fuchsia, orange and yellow
Orange, yellow, red, green, blue, purple with white belly
Dark green and red (Titan)
Cavern Crasher
Black with yellow stripes
Grey and red
Dark brown yellow and orange
Gold, yellow, orange, red and blue tongue
Dark Brown with luminous pink and blue (Titan)
Pale pink with blue tongue
Deadly Nadder
Blue, gold, and tan
Orange and green
Green and purple
Green, red and purple
Blue, purple and pink
Purple
Green and pink
Purple and dark green
Orange and teal
Green and yellow
Bright purple and yellow
Greyish green and pink
Dark purple and dark yellow
Turquoise and yellow (Book of Dragons)
Blue, yellow and pink
Light blue
Lilac
Green
Hints of cyan, pink, orange and green
Dark Green and purple (Titan)
Death Song
Green, blue, red, teal, and yellow
Devilish Dervish
Black
Egg Biter
Metallic blue, yellow and grey
Red and Yellow
Purple and bright green
Pale tan with purple, leaf green and sky blue swirls
Light pink with blue and yellow stripes and spots
Pink and purple (Titan Wing)
Fireworm
Bioluminescent gold
Red and orange
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Red and yellow
Scarlet and orange with blue spots
Neon Green and Neon Yellow
Green, blue and yellow (Titan)
Flame Whipper
Cyan, orange, red and green
Turquoise, red and black
Green, brown and black
Cyan, yellow and black
Sky blue, purple and black
Flightmare
Bioluminescent neon blue and light blue
Orange and pink
Translucent, ghostly green
Glistening pink, blue and orange
Navy bue with luminous light blue hints
Purple (Titan)
Foreverwing
Woody Brown and Green
Gobsucker
Greyish green and red
Blue, navy blue and yellow
Yellow, metallic pale green with hints of yellow and navy blue
Orange, pale pink, lilac and petunia
Shades of purple
Pink, violet and greyish green (Titan)
Grapple Grounder
Black, red and gold
Red, Yellow and White
Green
Brown and green
Orange
Black and green (Titan)
Groncicle
Gronckle
Brown
Lime green
Reddish brown; Blue and green; Green and orange; Yellow; Purple; Brown and orange
Dark green and white
Light pink and yellow
Greyish green and yellow
Purple and blue
Green, tan and orange (Book of Dragons)
Purple; Orange; Blue
Dark green
Red and green
Orange and green
Pink
Yellow and orange
Black and red (Titan)
Green and light blue
Green, brown, yellow, orange and white bumps
Hackatoo
Woody brown and light red
Purple and yellow
Light blue and hints of purple
Blue and yellow (Titan)
Hideous Zippleback
Green and maroon
Blue and red
Pink and brown
Orange and brown
Light blue, orange and yellow
Pink, purple and blue
Gray, yellow , green and purple
Green and blue
Maroon and black
Greyish green, blue and purple (Titan)
Hobblegrunt
Changes color based on emotions. Spots remain blue
Brown and yellow
Blue and purple (Titan)
Hotburple
Brown and dark brown
Green and brown
Gray, pink, blue, yellow, green and brown
Light and dark blue (Titan)
Moldruffle
Black and orange
Purple
Dark navy blue, white and red
Orange and blue (Titan)
Monstrous Nightmare
Black and orange
Purple and yellow
Light blue, orange and purple
Yellow and light green
Lime green; Cyan; Pink
Orange, Pink and Green
Faded cyan with hints of yellow, pink and orange
Pure red (Book of Dragons)
Blue and yellow
Pink and purple
Blue and red
Yellow and green
Purple and pink
Yellow and pink
Orange, fuchsia and green
Funchsia and yellow
Lime green and orange
Red and black (Titan)
Red and black (Titan)
Pink, purple and yellow
Blue, red and black
Mudraker
Turquoise
Dark blue
Night Fury
Jet Black
Jet black with blue spikes (Alpha)
Night Terror
White
Emerald green with some yellow
Dark grey and orange
Dark red
White and black
Grey and blue (Titan)
Prickleboggle
Green and red
Orange and blue (Titan)
Purple Death
Purple
Raincutter
Purple
Green
Greyish green and orange with dark purple wings
Light blue and purple
Gray and red
Luminous blue and lilac
Light blue, pink and dark green (Titan)
Razorwhip
Metallic grey
Light grey
Bronze
Jade with some white
Dark violet and silver
Dark brown and grey (Titan)
Red Death
Grey and red
Grey and green
Rumblehorn
Sand Wraith
Brown
Purple and brown
Shades of purple
Red and yellow (Titan)
Gold and Lime Green
Scauldron
Sea Green
Red
Gray (just when their body is drying out)
Turqoise
Dark green and hints of blue
Teal and light blue
Purple, turquoise and light blue (Titan)
Screaming Death
White
Scuttleclaw
Blue and yellow
Red and blue
Green and red
Yellow and light green
Purple
Turquoise, white and red
Dark green and brown (Titan)
Seashocker
Sea blue
Teal
Black and sky blue
Purple and yellow
Purplish blue, pale yellow with shiny light blue splotches
Yellow and pink with light blue spikes (Titan)
Silver Phantom
Purple and white
Singetail
Orange and maroon
Brown and red
Pink and purple (Titan Wing)
Green
Yellow
Shadow Wing
Dark Teal and Red (Titan Wing)
Shivertooth
Blue and purple
Green, red and blue
Green and purple
Metallic grey and blue (Titan)
Shockjaw
Green and Blue
Green, white and orange
Luminous shades of Blue
Luminous Teal, Cyan and Pink
Dark blue and purple (Titan)
Shovelhelm
Cyan, dark green and brown
Green, navy blue and black
Light greyish-blue and brown
Dark red and tan
Lime grey and purple
Orange, blue and pink
Brown, blue and yellow (Titan)
Skrill
Dark gray and purple
Purple and dark blue
Shades of blue
Orange and yellow
Purplish brown and green
Light blue
Pink, purple, green, blue with white spots
Dark blue (Titan)
Slithersong
Blue and black
Neon pink and baby blue
Turquoise, white and red
Red, yellow, blue, and grey (Titan)
Sliquifier
Green and purple
Blue and pink
Yellow, Orange, Red and Dark Brown
Yellow, blue and green (Titan)
Small Shadow
Shades of blue
Green and cyan
Red, yellow, blue and green (Titan)
Smothering Smokebreath
Bluish gray
Pale white and violet
Brownish gray (with painted white skulls)
Pale gray
Purplish gray
Pale blue, purple and turquoise (Titan)
Snafflefang
Teal and red
Jade and orange
Blue and orange
Dark and light purple
Yellow, pink and orange (Titan)
Snaptrapper
Lime green
Orange, green and yellow
Grey and green
Brown and light green
Dark green with black spots
Brownish red
Brown, pink and black (Titan)
Snifflehunch
Blue, red and lime green
Turquoise and lime green
Blue and lime green
Orange and lime green
Shades of purple
Blue, cyan and white (Titan)
Snow Wraith
Snowy white
Pale tan and white
Glistening blue
Lilac and orange
White and light blue
Turquoise with whitish tan spikes (Titan)
Speed Stinger
Lime green
Lime green with red stripes
Green, blue, yellow and red
Light blue and pink
Yellow with red stripes, purple
Light green, brown and yellow.
Magenta, dark blue and yellow (Titan Wing)
Stormcutter
Bronze and reddish with blue
Violet and purple
Shades of blue
Green and yellow
Dark brown and pale pink (Titan)
Submaripper
Sweet Death
Yellow
Orange and green
Green and brown
Dark bronze (with yellow paint)
Red and white (Titan)
Sword Stealer
Yellow and grey
Black and green
Yellow (Baby)
Purple and green
Red, black and yellow
Golden and blue (Titan Wing)
Terrible Terror
Rust and green
Red and gold
Green (Book of Dragons)
Yellow
Cyan
Purple
Orange and red
Green and red
Purple and blue
Green and red
Dark red (Titan)
Thornridge
Red and dark green
Green and orange
Shiny sliver reflecting hints of yellow, blue and purple
Cyan and green with white spots
Lime green and purple (Titan)
Threadtail
Purple
Blue and green
Cyan and yellow
Purple with green spots
Orange and grey (Titan)
Thunderclaw
Grey
Blue, green and pink
Brownish red and tan
Purple and pink
Red and tan
Yellow and dark green (Titan)
Thunderdrum
Dark blue
Teal
Violet
Light blue
Dark violet
Pale purple
Purple and turquoise
Yellow and Orange
Light blue and turquoise (Titan)
Thunderpede
Red and grey with blue limbs
Yellow and white
Black, grey and glowing lava red
Brown (Titan)
Dark gray and Rainbow
Timberjack
Brown and tan
Redish and scarlet
Greyish brown
Brown and green
Red with orange
Green (Titan)
Tide Glider
Triple Stryke
Dark brown and yellow
Dark orange and white
Neon green and light blue with specks of yellow
Black and gray
Dark orange, pale tan and bronze (Titan)
Typhoomerang
Whispering Death
Dark green and red
Dark blue and red
Dark green and pink
Blue and orange
Reddish brown and teal
Dark blue and red
Light blue
Purple and brown (Titan)
Windgnasher
Lime green and dark green
Orange and brown
Luminous blue with white highlights
Pink and green (Titan)
Windstriker
Grey and yellow
Blue and yellow
Yellow and grey
Blue and pink
Blue and white (Titan)
Windwalker
Red with yellow belly
Purple with blue spikes
Woolly Howl
Pale brown
White, green and and yellow and red ornaments
White and shades of green
White and purple (Titan)