So long as colors don't look disorganized (IE, in drab cascades gathering in abundance), I'm of the train of thought that all colors are contributing in some way, whether it be the inherently passive tones of green or the more commanding, deeper tones of dark purple and blue (or just darker, deeper colors, to make a more general statement). IMHO. So it's sort of a quagmire for me to choose. If I had to choose one, though, it'd be Indigo, if only because of the aforementioned reasons.
But colors themselves to me are visible emotions, just as music may be equal to audible emotions; both are used in art forms. Either way, I've always appreciated colors immensely.