Pretty soaps!
My guess is that, once you have sub-divided your numerous colours, you pour very thin layers at a time, especially when the layer onto which you pour is not too liquid to mute the colours.
I figure you can get the best results by correctly planning out the colour palettes well. The colours need to contrast each other sufficiently to have the desired effect, even when you are mixing different shades of the same colour.
In some cases you might slightly tilt the mold to one side.