Red is one of the primary colors, which means it cannot be made from any other color. Green, for example, is a combination of yellow and blue, the other two primary colors. Spectral red, the color found in the rainbow is a pure red. Shades of red are developed by mixing in other colors, white and black. If you would like to darken the color of red paint, it's a simple but tricky process. Tricky because you're creating a color that might be difficult to reproduce. Simple because you're adding only one color to the red.
Measure 4 tbsp. of the red paint into a disposable container. Add 1/4 tsp. of black paint and stir well with the craft stick. Paint the darker red paint on poster board. Close the container. Let it dry because most paints dry a darker color. Mark next to the paint that it was 1 to 48 or one part black paint to 48 parts red paint. One tablespoon equals three teaspoons. One teaspoon equals four quarter-teaspoons. So four tablespoons equals 48 quarter-teaspoons.
Measure another 4 tbsp. of red paint into another container. Add 1/2 tsp. of black paint and stir well. Paint the poster board. Mark next to that it was 1 to 24.
Continue adding more black paint by 1/4 tsp. to 4 tbsp. of red paint until you reach the color you want. Different surfaces will show colors differently. Paint a small swatch of the final red color on the surface that will be painted. Let it dry. If it dries to the color you want, you're finished. If it's too light, go back and make another batch but add only an additional 1/8 tsp. of black paint. If it's too dark, use the last formula you used but subtract 1/8 tsp. of black paint. In other words if you used 3/4 tsp. — 6/8 tsp. — of black paint to 4 tbsp. of red, use 5/8 tsp. of black paint.