Beginners often find it difficult to judge both the strength and quality of the first colour to be applied. There are 2 reasons i) water based colours dry out lighter after being applied ii) it is difficult to judge a colour against a white background. Watercolours are built up in a series of overlaid washes or brush strokes. Do not despair, the first wash and tone that you put down may not be final, many further changes can be made on the paper itself. For instance, if a pale yellow wash is covered by a blue one, then the resultant colour will be green, and the tone darkened due to the presence of 2 layers. Also, a wash that is too light can be darkened by application of a second layer. This is the basis of monochromatic painting; a cool green (blue based) can be warmed with a yellow wash on top.