Once they are adults it's pretty easy to tell the sex, but not so easy when they are smaller.

Cresties undergo a pretty regular color change, from their normal dull color. It's called fire-ing up. It can happen in the evening when they are more active or when they are stressed. So yes, it's pretty normal.

As for selecting a crestie, I can't be of much help. Usually it's pretty normal to ask for pics of the gecko in it's normal and fired up state. That's probably the best way to go about it. Looking at the parents is another common thing to look at.

My babies have changed dramatically since they hatched. Dramatically, and it's normal for them to continue to change until they reach their adult colors.

Sheesh, now I can't tell if I've helped or just confused the situation. I hope it the first one.