Mud is that awful grayish or multicolored sludge that forms on your paper instead of your painting. All transparency is lost. Artists do use mud as a tool. More often, it simply appears where least wanted.
Use more pigment!
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1. Water or palette dirty.
Use clean water and clean your palette with every painting.
2. Not waiting for one wash to dry before you slap on another.
Try letting colors bleed together on your paper instead of mixing on your palette.
If you're a beginner - mix a maximum of three colors together to start, then let it dry completely before you add more paint.
3. Not using enough pigment.
Use enough water and pigment for strong colors.
4. Last but not least, student or low quality paints.
Low grade paints have very little pigment and a lot of filler or kaolin (clay or well, mud).
If you're painting with mostly mud, how could you not get muddy paintings!
Use good quality paints - I promise you'll use less and get much better results! Good brands are M. Graham, Daniel Smith, Schminke, etc.