My logic generally goes:
vision>pattern>color>variations in both
Some of the things I do evolve enough that it is hard to discern the original intent. I think that is why I am fascinated with the discipline of the birthday blocks, using someone else's colors, and trying to fit them into something that appeals to me.
Some things work great in 2 colors, but they require a strong design element. Others look great scrappy, with colors galore, and variations standing out mainly from color values.
I have seen portrait quilts which look photo-realistic, just done in light-medium-dark variations of the same one color. They are striking.
Like anything else in quilting, the rules are to do what is right for YOU!