This is a common problem among young artists I work with. When you have such a large array of colors to chose from and mix where do you stop. Because invariably using and mixing to many colors can and often does lead to a muddy look in your work. The painting looses its freshness.
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.