Two things you may want to look at when painting are:
1. Adding a small amount of a color to its complementary color will dull the color.
You may want to look at this discussion I wrote on my website.
2. Be aware that mixing colors that contain all three of the primary colors will dull the color. If you look at the complements...red and green... the red is primary and the green is made up from yellow and blue, the two other primaries. Limiting your mixing to only two primaries, for example, the blue, violet, red range will keep your colors cleaner and brighter.
Check your palette colors tubes to be sure your blue is not blue-green or your red is not slightly orange. What I have done is tested different combination of colors I have using a disposable palette. I label the samples and keep it as a reference.
As I re-read your question, you may also be talking about the loss of gloss of some of the colors. If you click the search button above in the main menu and type in "varnish", you will find several posts on that topic. Here is one of them.http://watersolubleoils.forumotion.com/t4-how-to-varnish-a-painting-completed-in-water-soluble-oils?highlight=varnish
Hope that helps.