A former student remembered that I say to use all six basic colors in each and every section of a painting: that is, every section bound by a hard edge. She successfully did this but was dissatisfied with the overly colorful look, verging on psychedelic.
So she is ready for the next step: the fine-tuning of how MUCH of each color to use and how much to blend. Just using the same amount of each color in each section makes for a colorful patchwork, and over mixing of the colors makes mud. So the key is the ultimate skill needed by watercolorists: being able to determine how much water is in three areas: the brush, in the paint, and on the paper, and how much pigment is on the brush.
Of course experience makes this happen, but first you must mentally pay attention to each part of this. Do you know how wet the paper is? That will determine how far the paint spreads. Are you conscious of how much water is on your brush? Best to keep a semi damp brush but not bring extra water to spread the color before you want to. And sometimes you think you are grabbing a glob of “dry” or paint with no water added, but instead a little water gets in there and you have closer to a wash.
This student was already great at seeing all the colors I asked her to include: she saw variations in an orange petal that added up to hundreds of colors so is past the idea of painting the petal a general orange, resulting in flat, boring paintings. But she was not making a judgment as to how much and exactly where the orange was duller, lighter, darker, or actually redder than orange, or more yellow than orange. She could even see green and purple in a generally orange section.
Now she will try to use smaller amounts of all the colors, so as to not put too much. You can always add more. And she will be more careful as to where she places the alternate colors. This is the hard work of seeing the world as it really is, not what it looked like at first glance.
Don’t forget, too, the three uses of the brush: delivery of water and pigment, then mixing with a clean brush, and later removing with a clean dryish brush (lifting), if needed.
This next step to excellence is hard like all steps to improvement because you might feel you have come so far already, that you don’t want to hear about yet more ways to get better. So if you feel that way, rest on your laurels until you become dissatisfied with your work and want to improve. After the above lesson, there are many many more ways to get better. You can stop at any time and just paint the way you can, enjoying it but not needed to improve. That might become boring and then you can go back to more advanced lessons.