Most women will prefer a ladies cut shirt to a standard uni-sex t-shirt.
Dark colors are usually a fair bit more popular than light colors, but it does depend on your market too. A friend of mine that runs
www.syswear.com (geek and gamer shirts) gets all of 7-8% of his sales on white and ash combined, and the rest on dark colors (red, navy, and especially black)... other markets won't be quite as clear cut, but darker colors are going to be more popular. It also depends on the designs and what works well on what color, of course.
Finally, it also depends on your printing method. If you're using a partially on-demand method like plastisol, you can offer more colors since you can print them as they are ordered. OTOH, if you're outsourcing traditional screen printing, you'll need to have shirt colors (and sizes) determined ahead of time so you would want to offer fewer colors.