To be fair, Walter Jones (Zack) was given a choice of which color to be. After thinking that there was no way in hell they would let him be Red, he chose Black so he could be a Black superhero/role model.

Thuy was also the second one cast for the Yellow ranger after the original quit when she saw how shitty they were getting paid (which would eventually lead to Austin, Walter, and Thuy quitting in the second season when contract negotiations fell through)