Spectrum can use two colours, have a play with it and you'll see. Certain colours work better than others.
Edit: Actually, I'll show what I mean. So, spectrums 2 colours are sort of like "Base" and "Highlight". I say sort of, because not all colours work. So here, black is the highlight and thus you don't get a highlight:
Whereas here, white is the highlight and a fair bit more noticeable:
But, like I said, it's quite finicky on what colours work together. And they don't always look good:
A lot of the time, there's very little difference in using a lot of colours as the highlight instead of white, but you can sometimes stumble across nice combos:
As is the case with costume creation, a lot of trial and error.