Intensity, also called chroma or saturation, refers to the brightness of a color. A color is at full intensity when not mixed with black or white - a pure hue. You can change the intensity of a color, making it duller or more neutral by adding gray to the color. You can also change the intensity of a color by adding its complement (this is the color found directly opposite on the traditional color wheel). When changing colors this way, the color produced is called a tone.
When you mix complementary colors together, you produce a dull tone. However, when you put complementary colors side by side, you increase their intensity. This effect is called simultaneous contrast — each color simultaneously intensifies the visual brightness of the other color.
Credit: University of Saskatchewan