Well first, I'll state that they were the biggest wastes of money ever, but I got caught up in NEEDING these things that mean very very little to the universe.
High school ring, fake rose quartz, since opal is my birthstone but it's not sturdy enough to be in a class ring (and hard to make fake, LOL).
College ring I got green, as our school colors were green and yellow, and I was on Crew and wanted to show spirit (to whom I'm not sure, as I got the ring for my pinky and it was tiny). Had crossed oars on it and everything.
But I see nothing wrong with just choosing a color, if that's what she wants.
Editing to mention....the problem with choosing a color that you like is that tastes tend to change. I know color tastes sure do, I loved purple in HS, adored bright orange in college, and now I like 'em all. So a choice based on birthstone or school colors is something that is forever (not totally true, as my college decided to change its colors 10 years after I was graduated), while a stone choice based on current fave color might not last beyond frosh year of college.