I wasn't sure of what colors to go with for Rhoda (big surprise). I always figured her hair was a dark brown, as were her eyes, and I had a vague idea of her skin tone, but the rest, I wasn't so sure.
Fortunately, I WAS sure of what colors to go with for Catalina, so I made Rhoda's color scheme something that would go well with that. End result? Peanut butter and jelly, basically. I imagine the creators of Futurama thought something similar when deciding on color schemes for Fry and Leela (I also like to imagine an actual sandwich was somewhere nearby when their hair colors were decided upon).