How do you prevent colors from going muddy?
When you paint from observation, and you carefully reproduce the colors you see in your
painting, the color will never look 'muddy'. (with color
i mean hue value and saturation). So colors themselves are never 'muddy'.
They only look
muddy when they don't express the light and material you want to describe.
If your colors look muddy: try to hit the colors in your subjet
more accurately. Compare colors in your painting to the subject and try to answer the
questions : does the color in the subject look redder/bluer/ yellower / darker / lighter /
more or less saturated etc than in my painting, and change if necessary.. Then at some
point the painting won't look muddy anymore..
A good book on color mixing helps (eg. Helen van Wyck.)
I feel that I have less difficulty in mixing the color I want using high quality paints
like Old Holland. This paint is more expensive, but you don't need the most expensive
pigments. Most colors also come in a cheaper A-series alternative.