I have been trying different paints (shades of yellow) for my dining area and have a collection of samples. Instead of throwing them away, I mixed some with the white primer and that resulted in using less topcoat paint. You don't always need a primer, but I had the walls re-textured which seemed to absorb the paint and so needed to paint with a primer first. You only need enough color to tint the primer.
I also painted the interior of my garage and after watching a show about recycling paint dumped unused paint into a large bucket and used that. When mixing paint together separate warm colors from cool colors.