When we start to learn to paint, we just do what is the more logical way to dark a color and this is just by adding black to any color, but the problem is that the black color is too strong and if you are not a profesional painter with so many years of experience the only thing you will get is kills the other pigments making them dirty.
Here is the rule to dark any color, it is based in the color wheel and the complementary colors to make it so easy to understand.
What are the complementary colors ?
orange – blue, red – green, violet – yellow, then when we want to dark the yellow we add violet, more violet more darker would be the yellow, we will do the same with the red by adding green and the orange by adding blue, this colors are the base for all the other colors we create.
Look a color wheel and find the opposite color, remember the darker color will dark the lighter color.