I don’t know how to draw these things until I know what they are. There’s apparently zero consensus, almost every comment is different. The only thing I know is that I’m pretty certain they’re not leaves as one person suggested.
Also, finding specific fanart interpretations is a bit difficult as I don’t watch and save/follow any artwork of Spike.
Ear-fins.
You’re probably right, they’re modeled after the head-fins you sometimes find on art of Eastern dragons, but for animation purposes here they act like rabbit ears. They give the artists an extra opportunity to express his moods.
It’s pretty normal for body parts to change colour with age. Happens to a lot of species with different parts (including humans)
Edited
Is that head Cannon or based off of something from a real animal?
Edited
That’s strange, because now it looks like they changed colors if they’re ears. (Could be animation color error though).