Blooms, washbacks, backflow, cauliflowers … whatever you call it when you’re painting wet-into-wet watercolour and the colour you just applied pushes out the one already down, rather than making friends and sitting with it. There’s one rule to avoiding it (or to use if you deliberately want to get this result, much as watercolour purists might shudder at the thought). In the words of that skilled Australian watercolourist John Lovett:
If you are painting a soft edge into a wet wash, make sure there is more pigment in the color you are applying than is in the underlying wash or obvious blooms will be created.”
Source: John Lovett, Watercolour Edges
So how do you know whether you’ve got more pigment or not? Like everything, practice. It starts by deliberately considering it, and eventually it becomes ingrained knowledge, instinctive. If in doubt, add more pigment (“thick paint”). Or pull some of the water from the brush hairs by holding a piece of paper towel to the ferrule end of the hairs, a tip the artist Katie Lee taught me.