Purchase white oil-based paint. Look for the words "oil-based" or "alkyd" on the front of the can.
Buy empty paint cans. Many home improvement stores sell empty paint cans with lids. If you don't have any luck there, you can try your local hobby or craft store. You will be mixing and storing the white oil-based paint and counter paint in these cans.
Pour the counter paint into the empty paint can. The amount of counter paint you pour into the empty can depends on how much white paint you want to add to it later. If you aren't sure, fill the can halfway. This leaves you plenty of room to add and stir the white paint.
Add white oil-based paint. Add a little bit of white paint at a time and then stir the colors thoroughly together. Keep adding white paint until you reach your desired color.