You could show her the choices on the fabric websites and let her see the variety available. The lighter colors to me still carry a "girly" feel to so many of them.
I try to use an animal or fun type print that is bright and colorful. After many many washings you want the colors to stick around and not be puny looking.
For a boy, the zoo look is great, construction items, dinosaurs, cars and what not.
No matter what the color, the love it's made with is the most important