Color theory doesn’t happen intrinsically. It isn’t usually a gift we just “know”. It would be my guess all of the famous artists throughout time have encountered challenges with color. It takes some area of knowledge and then a lot of practice. The real challenge is mixing colors that you desire for your work. When experimenting with color searching for the right hue, it is easy to keep adding and adding until you have a great mound of mud. Something you can’t use for anything. This is what color does. It will inhibit you from moving on when the right color can’t be found.
To avoid that huge pile of mud, the thing to do is mix very small quantities. Don’t keep adding to that original pile of color if it isn’t right. Adding more color probably won’t make it right thus the mud. If it isn’t the right color, make another attempt in a fresh place on your palette. Now, with that said, if the color only needs to be lightened or dulled, add white or black or whatever color close to those to change the brightness or dullness. This is easily done. But, make those changes in small increments, its easy to go too far then the color is compromised.
The best exercise you can do to create color is to decipher everything you see mentally for color. No matter where you are you don’t need paint and a brush to do this. Go over in your mind what you would mix for that color you are looking at. People may wonder what you are doing looking intently at something and thinking, but that’s ok, we’re artists. Mental exercises can be done anywhere and you don’t need supplies. If you want you can take notes to try when you do have a brush and paint. See if your formulas work.
Another highly recommended thing is to of course, learn the color wheel. The location of color on the wheel is very important and a must know if you are going to be using a limited palette. Basically, all you need are the primaries, white and black. From there you can go anywhere with color. Make notes and certainly make yourself color charts for color saturation and value. These are invaluable for both reference, and practice of the mixing. Proportions of color are very important as well. It all takes practice and basic knowledge of the wheel.
If you are working in color at all, these are things that would benefit you to know and practice. Just remember to take it slow and mix small amounts so you don’t get that monstrous pile of brown mud.