The two extreme tones or values are black (very dark) and white (very light). Recognizing the tone or value of a color, rather than the hue, is important to a painter because successful paintings have tonal contrast in them, or a range of values.
A painting with only mid-tones risks being flat and dull. Value or tonal contrast creates visual interest or excitement in a painting. A high-key painting is one in which the contrasts in value or tone are extreme, from black right through the range of mid-tones down to white. A low-key painting is one in which the tonal range is narrower.
To familiarize yourself with tone and value, paint a gray scale using black and white paint. This has white at the one end, black at the other, and a range of tones in between. Print this art worksheet on a sheet of watercolor paper or card for an a quick, easy-to-use grid. Start with a block of white and a block of black, and gradually work your way towards a gray scale with nine tones.
Now repeat the exercise, using different hues to create value scales for the colors you use frequently.