Isn't it ironic that the more tightly you control what the paint does, the less it looks like watercolor? As watercolor painters, we aspire to a fine balance. We strive to give the paint room to assert its fluidity while keeping the results within acceptable bounds.
Stroke by stroke, we try to establish the widest possible range within which whatever we do will be "perfect enough". Having clear intentions is a big help, since decisively applied paint looks fresh and clear, while tentative strokes look uncertain. Having a definite understanding of the requirements of the passage you are about to paint allows you to make a bold statement with the brush - one that does not need correction.
In Kate Barber's wonderfully loose street scene nothing has been rubbed out, covered over, re-done or otherwise corrected. Instead of making the paint conform to a narrow scope of correctness, Kate established a very wide range of what would be acceptable. Take a look at the sunny side of the street. Those buildings comprise two or three layers that progressed from light to middle to dark. Try to visualize what the first layer - the lights - looked like before the middle value shapes and the darks were applied. The artist knew that as long as the shapes were vertical and roughly rectangular they would work just fine. She had a clear vision of what the next two layers would do to ensure that those shapes would read as buildings, so she could make the lights boldly and leave them alone. We viewers get to see the efficiency with which the buildings were created, since at least some of each layer remains visible.
Most of you took home a photo or two from class. Choose one of them, or use one of these, below, and do the work you need to do to understand the role each layer will play in creating the illusion you want. Some kind of preliminary study is probably necessary. Remember, the study doesn't have to be a handsome product to do its job. you'll learn just as much from a quick, rough draft as you would from a subtle, perfect little rendition. More, probably.
The aim is to discover the guidelines that describe what must be true for your treatment to be "perfect enough". Keep them simple - no more than three requirements - and keep them abstract. Kate Barber's guidelines for those buildings were very simple (vertical rectangles), and they were based on form, not content. The sentence I like to use is, "As long as what I make is _____, _____, and ____, it will work".
When you know what you need to know, paint a picture. Have fun!