At the age you're talking about, the color difference would be easy to see in pure bred birds. Males are much lighter, females are darker. Go for non-existant combs, and a dark wash of color down the front of the legs is a good female sign. Look around on the barred rock threads, and this section to see the color differences, it's usually easy to see.
Thank you! lots of good info (about Roos, too!) I think I will be able to make an informed decision, hopefully on all three. My chicken journey begins on Friday!!!