almost definitely fur colour! that's the part that'll be more or less constant, whereas you might put them in various different outfits and colours and styles later.

that's a difficult question to answer.

simply put: yes, it can happen. to explain further, DID is a covert disorder that the person is not supposed to be aware of in 90% of cases. (leaving the 10% for the ones who do happen to be overt, which are every bit as valid.) in many cases, DID begins in childhood but is hidden until adulthood. to only be discovering the disorder now, or to only be seeing the disruptions now, is extremely common. especially if you have recently moved away from an abusive household or other distressing event.

it's kinda the same process as any other aspect of character design, I guess? like, thinking about the vibes you wanna convey, things you want to express about yourself through your sona's design, maybe some aspirational elements that you think are cool/fun but wouldn't necessarily wanna try IRL, that sorta thing.

colour theory is weird and subjective, and all of this stuff is sorta personal when it comes to fursonas so I can't really tell you an exact way to approach it, but I guess the biggest thing is deciding between a critter with natural fur colours (browns/greys/etc) vs more vibrant ones? it can kinda convey the "whimsy level" of the character based on where you land between darker, muted natural tones (which are often meant to blend in with surroundings and thus tend to make a character feel "ordinary") to bright, cartoony, eye-catching ones. you can also do a mix if you want, like a character who's mostly dark grey or tan but has blue or red accents for example.