One of the most common things I hear from beginning painters is "How do I avoid muddy colors. Hopefully, these tips will help you keep clean colors.
Be Organized and Clean When Painting - Avoid Muddy Colors
Make sure that you work with clean brushes. Don't leave residue from other colors. Use one brush for light colors and one for darks.
You should get into the habit of laying out your palette of colors in the same order and place them at the edge of the palette to leave plenty of room for mixing. If you squeeze out the paint in long lines instead of puddles, it is easier to take paint from the end of the line, leaving the rest of the paint clean. Use a palette knife to dip into the paint instead of a brush. Then wipe your palette knife clean. Continually wipe your palette clean so that you keep your mixing area clean. Alcohol works well to clean the palette. Remove any dried paint. I will scrape with a razor (I use a glass palette) away any dried paint around each pile of color to keep colors separated from the next. Disposable palettes definitely will help to keep a clean mixing area. Don't be stingy and not throw it away when it becomes over used. That is the beauty of them, you can throw them away after each painting session.
Clean Saturated Colors
I've heard people say that you shouldn't mix more than three colors together at a time. I don't go along with this school of thought. Often I will have mixed a gray or used my left over paint from the day before. If I need to adjust the temperature or color of the gray, I will add another color to it. Once you know your colors well, you can do this. Do you paint straight from the tube if you want pure, bright color. But don't overdo it. You can create chaos in your painting with too much saturated colors.
Use Like Values
When mixing one color into another keep the values the same or very similar. When you mix a darker value color into a lighter color or vise versa, they don't mix well and can create muddy colors.