Dirty water and dirty brushes
Some years ago, I was visiting the Open Studio (Santa Cruz Open Studios) of two Santa Cruz colorists. While I was there, someone asked one of the artists, what was her secret to good color? "Dirty brushes," was the answer. The second artist laughed and said, "Mine's dirty water!" (We are talking about acrylics here.)
There's a reason for this.... As you paint, using the same brushes and water, you'll get a little of each color into each other color, until — presto! They're all related. They share the same characteristics - they have the same DNA (to use a familial metaphor). And you get color harmony.
The moral of this tale is: use the same brushes and water — and don't think you need to "freshen" the water. I use the same water for days... honest! It works really well....