The author mentioned keeping a clean workspace. As part of this, be cognizant of remnant color in your brush or that your brush picks up if you are painting over wet paint. If you have a lot of problems with this (I did), get yourself an arsenal of cheap brushes and force yourself to allow a couple days drying time before coming back to work in lighter and darker shadows in the barely wet paint on you canvas.

Last point: learn the difference mixing with between titanium and zinc white because they give totally different results.

As an artist and art instructor, I have found that helping beginner oil painters break the habit of over touching, or worrying their painting is also very helpful for keeping colour from becoming muddy and dull. Try a simple exercise of limiting yourself to three touches of a loaded brush to your painting. Then move back to your palette to pick up more paint or clean your brush.

It might be hard at first, but the clarity of your colour will definitely improve.
Kindrie Grove´s last blog ..Early Stage Painting Blues?