A keen sense of color is critical for a visual artist. An undesirable undertone, too much white, too much black, for example, can shift the viewer's perception and deliver a message far from what the artist intends. If you feel your color sensitivity needs sharpening, a good color theory course is a place to start. Of course, experience, trial-and-error and practice cannot be overestimated..
Decide on a shade to start with. If you choose blue, for example, it may be best to start with a medium tone. This will make it easier to see the color changes that occur. If you start with red, you can achieve an amazing range of colors, depending on which color or colors you add. To red, add white to achieve pink, yellow to end up with orange, or blue to get purple.
Add another color to your base color. Add just a little at a time until you have achieved just the hue you want. To turn purple to lilac, add white. To change black to gray, add white. To achieve tan or beige, add white to brown. If you add too much color at once you may have to toss that batch and start over. If you are adding black or white to any color, be especially careful to add in small increments.
Add other colors, if necessary. If you add white to blue, for example, and you decide you prefer a grayer blue, add black in small increments until you achieve the shade you desire. To achieve eggplant, you can add red to blue, which results in purple, then add brown paint until you get the eggplant tone you are striving for. Ochre can be achieved by adding yellow to red, and then adding brown or a little black.