For each pixel in the costume, the effects say how much to add to that pixel's H, S, or V. So, a positive color effect causes a blue shift, like a star that's moving, umm, toward us, I think? But hues wrap around, so if the pixel starts out blue and you use a positive color effect, it could become red.
For the other two, bear in mind that a full-intensity spectral color has both S=100 and V=100, and you can't get any higher than that, so if your initial costume is a rainbow, it only makes sense to use negative effect values. But if some pixels are other than spectral colors, then positive saturation or brightness effect can make them more colorful.
I think that's all right...