Whenever you activate a preset, you'll see that preset's name in the "Now Playing" bar, in this case "Rainbow". If you look to the right of the bar, you'll see one of three icons. (Brightness, Saturation, or Volume). When you tap it, you'll see a radial menu:
Drag the knob on the slider to adjust the overall brightness (a multiplier applied to all the colors in that preset at once). You can also tap on the saturation or volume icons to access those controls as well. Reducing the overall saturation will make any colors in that preset look "whiter" or more faded. Whichever control you select last will stay visible on the Now Playing bar even after you close the radial menu (by tapping anywhere outside the big circle).
If you're just looking for a quick adjustment in the future, instead of tapping, just press down on the brightness, saturation, or volume icon in the Now Playing bar and you'll see the radial menu open immediately on touch-down. Drag up and to the left and you'll see the slider follow your finger dynamically. Release at any time and that value will lock in while the radial menu closes.