How light or dark a colour appears is called Value. A lighter colour has a higher value than a dark colour. In order to see lines, shapes, different colours or textures, there needs to be value contrast. A highly contrasting art piece tends to create excitement and be more dramatic and attracts our eye to it. A low value contrast is more subtle and understated.
A variation in value can create a focal point and can be used to emphasize something in particular. To create the illusion of depth, use a gradation of value. Thus, changes in value create three dimensions in a work, such as shading the sides of a square to give it depth and appear to be a cube.
Some colours have a generally low value, such as violet. By adding white to violet, you can create a range of values. Yellow always has a high value as adding black to it soon changes it from yellow to a different colour. Mixing white to a colour is called a tint and mixing black to a colour is called a shade.
The value of a colour depends on the the colours around it. Using a red coloured piece of plastic will help you to see the value of the colours you are using, without the colour interfering. (In the quilting world they have a tool called a Ruby Beholder for this purpose.)
The FIVE Design Elements are: Colour, Line, Texture, Shape and Value.