When an artist refers to an area in a painting as looking chalky or muddy, the problem is usually color temperature.
“Chalky” refers to white without a specific and appropriate color temperature and “muddy” is probably a darker value without a color temperature. The problem is most likely paint without an identity – something akin to limbo, neither here nor there. This does not mean every bit of paint on a canvas needs to resemble a color wheel of primaries. Just a nudge here and there will identify a color as warmer or cooler.
Highlights and areas of very light values should have variation and interest. A touch of viridian or orange can do wonders for highlights, and larger areas of light value need to have an identity other than “white”. They need to be part of the overall pattern of color temperature but still be descriptive and interesting.
The problem with “mud” usually begins on the palette, but over-brushing an area of a painting will also mix the paint into one non-descriptive mess. The problem is not because the artist mixed more than three colors together on the palette. This “rule” arose in an attempt to find an easy solution to non-descriptive mixtures. Perhaps it works for some, but I am in the camp of sometimes scraping paint on my palette into one big pile and using it (and this is the important part) if it is appropriate and useful. This mixture of multiple colors might need some adjustment for value and color – lighter, darker, warmer or cooler.
Neutral mixtures of paint (tans and greys) are important. They enhance and help identify color. All areas of a painting do not need exclamation points. Just learn to appreciate subtle changes in color and pay attention to the overall pattern of lighter and darker values and warmer and cooler colors.