Primer is an integral part of the painting process. It seals, adheres, protects and penetrates so that a finish coat of either latex or oil-based paint will last longer and look better. It's sometimes a good idea to tint primer, especially if you're using a very intense primary color over it in order to cut down the number of subsequent coats. Because primer is specifically formulated for different purposes, tinting it with latex paint may change the color, but will also change the properties of the primer--meaning it won't work as intended.
If you're using primer for the sole purpose of cutting down on the total number of coats of paint necessary for a drastic color change, then it is okay to use latex paint to tint the primer. You can only add latex paint to a water-based primer; if you add latex to oil- or shellac-based primer, the two won't mix together and you'll ruin both primer and paint. Since most primer is tinted white or off-white, adding colored latex paint will give you a pastel version of the color. Adding a very low-hiding color such as primary red or yellow to primer is likely to make the primer very thin as well.
A better and cheaper choice is to bring the primer to a paint store and ask them to tint it as close as possible to the finish paint color. They may charge you a small amount for the cost of the tint, but this will retain the integrity and hiding properties of the primer.
Tinting Primer for Walls and Trim
Tinting primer with latex that you can use for your interior walls and trim isn't a good idea, because primer is not designed to be resistant to dirt or scuff marks, and won't wash well. If you wanted to use it for an area that is unlikely to get much wear, such as the inside of a garage or basement, that would be fine.
Split Coat Primers
In the old days, when most trim, doors and cabinets were painted with oil-based paint, doing a "split coat" was common. This entailed doing one coat of primer, followed by a 50:50 mix of primer and enamel, then one or two coats of enamel. While this is no longer common practice, this would be a good compromise between having a properly applied primer coat, followed by a split coat of primer mixed with latex paint, then finished with latex paint.
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