Hi there! I hope it's okay if I chime in real fast~
I do believe it's okay to change opacity because in the rules at the bottom of the overview its says:

" *5 - if there are only two colors in the game(black and white or their analogues),  [...] (The same color with different alpha channels is not considered separate colors.)"

An alpha channel is a color component that represents the degree of transparency (or opacity) of a color!
I hope this helps :D