What brand of paint do you use?
I use 'Old Holland' oil paints. These have a very high percentage of pigments. This makes
them ideal for wet-into-wet painting, as opaque paints are as opaque as they get. Paints
behave as you expect, ie. paint in red somewhere, and the spot actually becomes red, and
doesn't dissolve into the paint that is already there. Also, when mixing the paints, they
behave more closely to what they should theoretically (ie. like in the color circle). As
opposed to less pigmented paints, which get dull (desaturated) easily when mixed. The high
concentration of pigments also allows me to use more medium when I want to.