Ooh! Good topic!
For RP, I tend to design characters according to significant personality traits, since I believe it grants a nifty "theme" and direction to their look. For instance, I'll make use of soft, cool and/or muted colors for calmer, controlled and quieter characters while using brighter, bolder ones for energetic, spontaneous and loud ones. Other times, I'll use markings to make statements about the character -- like my character, Oyoo, who has permanent cub spots meant to reference his more childish nature.
Otherwise, I just make what looks cool. Particularly for "sparkle" characters. Heh heh.