Mixing old paint is a great idea. You save money and prevent waste, using what you already have. Also, experimenting with old colors is a great way to experiment with your creative spark. To mix old paint correctly, you must first carefully read the label. There are two main types of paint: water-based (latex) or oil-based (alkyd). Only mix like kinds of paint with like kinds. Latex paint and oil paint create a clumpy mixture that will make a mess on your walls.
Open each can and examine the contents. Discard any paint that has brown or white spots on the surface. This is bacteria that will likely spread, once the paint is on the wall. Also discard lumpy paint. First stir for a few minutes with a paint paddle, but if the lumps remain, the paint is not good.
Strain the paint to remove any debris or dried paint. Place a metal mesh strainer over a clean can or plastic container. Pour the paint through and discard anything the strainer traps.
Play with color. Use a plastic spoon to pour spoonfuls of different types of paint and mix a test batch. For example, if you have blue paint and green paint, add a spoonful of green to the plate, then add a spoonful of blue and mix. If you want a bluer hue, add another spoonful of blue. Always add paint one spoonful at a time so you can track ratios. One spoonful of green and two spoonfuls of blue is a 1:2 green-to-blue ratio.
Mix the paint in a separate container according to your desired ratio (for example, one cup of green paint and two cups of blue paint).
Mix the paint thoroughly with a clean paddle. Be sure to scrape the sides and bottom of the can as you stir.