This is sometimes termed graduated wash. Basically, it is the tool to add interest
and variety in an area without drawing attention to it through contrast. To
use an overall flat wash then the result becomes dull and lifeless. Gradation
also applies to shape and texture. Gradated wash means to change from a warm
colour to a cool one or from a light tone to a dark one. Tonal and colour gradation
are particularly useful for large empty areas of space such as skies and foregrounds.
Gradation can also help to increase the sense of distance and space. By gradually
decreasing the size of objects, as they recede into the horizon, and by gradually
reducing the tonal contrast, you can create the perception that the scene recedes
into the distance. Where you need a gradated wash (viz where a sky becomes paler
lower down) then apply more water in the mix. Work quickly otherwise it will
start to dry out before you have accomplished the task. Keep your board at the
15o angle so that the washes run downwards, or on the diagonal to suggest swirling
clouds. As I have stated earlier water colour tends to dry lighter in tone so
beware and experiment first, then apply.