Triadic colors are colors that are equally spaced on the color wheel. A triadic color scheme is an artwork that uses triadic colors in a purposeful way.
On a color wheel that contains primary, secondary and tertiary colors, there are four types of triadic color combinations: red, blue and yellow; orange, green and violet; red-orange, yellow-green and blue violet; and red-violet, yellow-orange and blue-green.
Triadic color schemes give a mild contrast, similar to a split-complementary color scheme, and yet are harmonious.
One famous work by Andy Warhol is called "Shot Blue Marilyn," which depicts the head of Marilyn Monroe with bright yellow hair, light red skin and a blue background. She has dark-red lips and blue eyeshadow.
When constructing a color scheme, consider substituting your chosen three colors for the natural values of whatever you are painting or drawing. If you are using violet, green and orange, use violet for dark values, green for mid-tones and orange for highlights.