Here is my easy-peasy process:
1. Assemble watercolor paper or heavy card stock and all the paint colors you want to use and lay out some newspaper. Your paints can be matte, gloss, semi-gloss, craft or big cans of acrylic, metallics and glitter paints as well. Find your oldest brushes with the stiffest bristles.
2. Apply just a bit of the base color onto your paper using very little paint and leaving white spaces (paper on the left). Allow this first coat to dry or use a hair dryer to speed up the process.
3. Dip your stiffest paint brush into the next color and pounce it around on the paper.
Continue with all your colors and finish with your metallic paints. It is not necessary to dry your paint between each color because you will get some interesting results when colors overlap or blend or mix.
Troubleshooting: There are no mistake in this process so you become an expert the very first time you try this process. Too dark? Add a lighter color. Too light? Add a darker color. If you don't have paint brushes, cut up a car washing sponge and sponge on each color.
So whenever you can't find the perfect blend of colors for your image, head to your paint stash and create your own!