How do you make red darker?

There are two easy ways to make any color darker in value. Depending on what result you are looking for, either one will work for you.

The first way (and easiest to understand) is by adding black. You have to be careful, however, because it doesn't take much black to get a REALLY dark value. The result of this technique is a sort of "shadow" of the original color. This might be fine for the application you need, but it can also make a painting look like a cartoon.

The second way to make any color darker is to add some of that color's complement. A color's complement is easiest to see on a typical color wheel. In the case of red, its complement is green, but you have to be careful to fully understand what type of red you're dealing with. PURE red's complement is green, but a red that is on the yellow side will have a bluish-green complement, and a red that is on the blue side will have a yellowish-green complement. This technique gives a darker value that is also a different hue, but this may also suit your application if you're looking for a more interesting and nuanced color.