For paints (as opposed to theater lights), the hues red, yellow, and blue are the primary colors. These building blocks of color are often arranged on a color wheel. From these three colors, with the addition of white or black, it is theoretically possible to mix the full range of colors in the spectrum. Orange, green, and violet are the secondary colors, or those colors that stand mid-way between the primary colors.
How to Spin the Colour Wheel (Tate)
Look through this summary of influential color theorists and their impact on modern artists.