Your first impulse when darkening a paint color may be to simply add black. Most artists who have worked with paint know that adding black to a color will make it murky and gray, which is often an undesirable result. Consider other ways to darken a color without muddying the hue. Although the method of darkening the paint is always the same, the colors you use will depend on what hue you are trying to darken.
Find a tube of paint in the color you'd like to darken, and squirt a little of it from the tube. Use your paintbrush to spread the paint into a circle in the center of your palette.
Choose the appropriate color to darken the selected hue from your selection of paints. The color you choose will depend entirely upon what color you are trying to darken: If you are trying to darken red, use blue or brown. If you are trying to darken blue, use a darker blue or a black (one of the few times that black is appropriate). If you are trying to darken yellow, use a light shade of orange. If you are trying to darken a secondary color, use the darkest primary color that was used to create that secondary color. (For example, if you are trying to darken an orange, choose a red tube of paint.)
Squirt the darker color onto the palette near the big circle of color you already placed on the palette.
Dip your paintbrush into the darker color, and drag a very small section of the dark paint into the original circle of color on the palette. Note that you are trying to darken this color by degrees, so proceed with patience and prudence. Add as small an amount of the darker color as possible.
Mix the two colors together, and analyze the new color. If you wish to go darker, drag more of the darker color into the circle in the center.
Repeat step 4 and 5 until the paint in the center is the desired color.
If you get the color too dark, don't lighten it with white. Use the original color from the paint tube that you squirted onto the palette to lighten the new color in the center. For example, if you were trying to darken orange--you squirted orange onto the palette and made a circle of it in the middle. Then you added a small squirt of red next to the orange, and you dragged little bits of red paint into the orange paint, darkening it by degrees. To lighten that orange now, add more orange paint from the tube to get closer to your original color.