Here are a couple of exercises that will, hopefully, shed some light on the variables at work when we put wet paint onto wet paper.
To practice, try this exercise, in which the wash on the paper is constant, but the wetness of the brush varies:
§ Make a large, shiny wash (not dripping wet) of a pale color.
§ Now load a brush with a contrasting color, using lots of paint and very little water, and make a stroke into the initial wash.
§ Next, add a little more water and make another stroke.
§ Keep adding water and making test strokes until you lose control of the edge altogether. When the brush becomes wetter than the paper you will see the second color displacing the first, resulting in a bloom.
A variation on this exercise keeps the wetness of the brush constant and varies the wetness of the paper:
§ Make three 6x6” washes, one just damp, one quite shiny, and one dripping wet.
§ Now load the brush with plenty of pigment and very little water. Work quickly, so your washes don’t dry. Observe how the brush strokes look on the palette. (you should briefly be able to see the tracks of individual bristles before the stroke flows back together).
§ Make a short stroke in the center of each of your washes. Were the results what you expected?