You might think that tinting your paint is a specialised task best left to the professionals, but you can easily tint your own paints at home. It's best done when you are trying to use some old paint, and are not extremely particular about the final colour. For example, if you have some white paint around from painting the bedrooms, and you want to paint the playroom or a finished garage, you can add tints in tubes that are available at any paint store to get a more lively colour. Purchase tints that are compatible with the base of your paints, whether oil or latex.
Start with a light colour paint. It will be easier to tint. White is ideal to start with.
Add a bit of the selected tint to the starting colour. Don't add too much at first, as it can be hard to lighten a colour if you go too dark. If you begin with white, you will get a pastel of the tint colour (for example, red tint will yield pink, blue will yield baby blue). Colours other than white will combine to form an in-between colour. Yellow with a bluish tone will yield a form of green.
Mix up the paint well. Scoop from the bottom to make sure you are blending all the tint.
Evaluate your colour. Add more tint if you want. The more tint you add, the darker the colour will get. Adding a complementary colour (like purple to a yellow mixture) will make the paint more brown and neutral.
Paint a test area on the wall when done, and let it dry. Evaluate the result, and adjust your colour if needed. Colours will dry a bit darker than they appear when wet. Richer and darker colours will also appear more intense when they cover all four walls, so dampen the colour down by lightening or making more neutral if that is a concern.