Never had time for this until today, thanks for all the  people who mentioned me! (I'm still not very known... *sobs* [no, just kidding again])

I'll try my best to do everyone, so please tell me if I missed anyone:

Celine: Bright fiery red and gold.  

Moon Wolf: Pale white and grey-black

Poinsettia: Grape purple

Periwinke: That periwinkle color? idk?

Jaybells: A pretty swirl of sky blue, deep blue, cyan, and white

Hawkstar: Bronze and silver

Sine: Pure golden

Pangolin: Dull yellow, sort of earthy yellow color?

Lord Entropy: Crimson and black 

Ultimatium: A swirl of black and white and green

Scuttles: Emerald green

Wordsong: The color of old parchment, and black

Writing: Soft orange and shades of greyish black

Artemis: Purple, magenta, white, and pink. 

Oh dear, I have to go! I think I did everyone currently active, at least? I'll add on later, if there is a later...

Scuttles: dusty sepia
Reuby Moonnight: black and red
Rora: red
Chaser and Jay: different shades of blue
Interrobang: white and silver
Darkvine: black and green
Endless_Parodies: reflective silver and light blue
Sinusoidal: sepia and burgundy
Periwinkle: Light blue, periwinkle, lavender, and pink
Lyric: golden
Rainbow: every color, basically
Hawkstar: sepia
Sterling: silver
Jaybells: blue and pink
Writing: black and silver
Wafflecat: golden and a tabby cat's fur
Poinsettia: red and turquoise
Spring Calamity: bright green

I'll try to think of some more in the future!

Here's a few! I don't do this too much because I tend to get images in my head rather than colors.

Celine: so here I don't really picture a color but instead the specific image of the light from a candle illuminating a dark room and maybe some purple 

Zealatom: bright green or I get an image of a boy in a gray t-shirt and jeans, leaning back in his chair, feet up on his desk, and looking in the direction of but just past the camera 

the antiquarian: a gold suit of armor. Gold or bronze

Reuby: a moon that has become a "light red" - not pink but not as intense as what the moon will sometimes look like in a total lunar eclipse

Seadragon: sort of like the good dragon from Shang-Chi and the Legend of the Ten Rings. A light teal-ish blue with a scaly texture 

Poinsettia: a young woman with olive skin, long brown hair, and brown eyes wearing a red velvety dress. She is sitting in a dark room illuminated by candles (guess she's friends with Celine??) and smiling at the camera. Sometimes in this image she is also surrounded by poinsettias and/or is sitting next to a Christmas tree, but the lighting of the image is not that good so it's hard to tell

Amethyst: a purple-ish aura. If I had to to picture her as a person I'd say she's got similar skin, hair, and eyes to Poinsettia but looks a little younger. She's got her hair in pigtails and has put a white bandanna in it. She's got pink rosy cheeks. She is giving the camera a big grin and behind her there are mountains and trees and the sky is lavender. There's also a river but it doesn't show in the image, I just know it's there

pangolin: bronze or just an image of a cute pangolin

Hawkstar: a mix of bronze and gold or maybe a hawk flying in the sky 

Maybe I'll do more later if anyone's interested in hearing what images come to mind when I think of them