One of the other issues any artist faces when painting in oils, acrylics, watercolor, etc. is the paints they use. Even though I know several artists who work with one red, one yellow, one blue, and white...I have chosen a double palette. My oil palette includes Cad. Lemon...Cad. Yellow...Alizarin Crimson...Cad. Red Med...Ultramarine Blue...Phtalo Blue...and White.
This will keep the color harmony in tact, but of course, there are other things to know (which I am still learning) and that is the attributes of color. An artist must learn to identify the root color of any color they see in the landscape and then determine if it has been diluted or "grayed down" and where it falls on the value scale. And that root color is always going to be one of the (12) colors on the color wheel.
It sounds so easy until you find yourself staring at a blank canvas. The only way to learn these things is to actually do them. Painting "en plein air" is the best way, but that is sometimes impossible, especially if you are in a populated area or there is no place to set up an easel. I am still amazed that the human eye can see so much more than a camera can pick up.
Never the less, the only way to progress is to actually pick up the brush or palette knife and actually do something. And of course, you need to tell your internal critic to be quiet while you work.
I have now painted approximately (10) paintings with a palette knife and I am told that the first (100) are the real warm up. So don't be discouraged, but join me as I press on towards my first (100). And there you have it...just a few more things to think about.