Another reason that a color might appear muddy besides over mixing is, if it is too cool or too hot for the spot. Examine your work and gauge whether the color in question needs to be wormed up accordingly or cooled down. A lot of the times that will fix the problem.
In the example below I painted a portrait using pure color as much as possible: Cadmium reds and yellows, alizarine crimson, and neutral grays..and the effect is a fresher more spontaneous painting. This pure color use was what made impressionists of 19th century so famous. I was furthermore mindful of the light hitting the subject and adjusted accordingly when painting her face. Remember warm light always produces cool shadows, while cool light produces warm ones. Happy painting.
"Girl with cat" Oil on board |