Certain colors do not mix well. The underlying pigments clash with one another and when mixes will turn muddy brown or gray. You can have a red with purple under tones and a red with yellow/green undertone that when mixed together will give you ghastly results. Do not mix complimentary colors unless you are trying to dull a color down, but do use them side by side to made your colors pop, like yellow next to purple. Get a color wheel at the art store, it will be of great help. If you want to take it further, a small watercolor set will show you what different colors do when mixed or sit next to each other. It is quite a science and one we normally do not pay too much attention to. Over time color mixing just becomes second nature.
When a color is translucent and is laid over another translucent color, it is the same as mixing colors.
I don't know if this is at all part of your problem, but this information might help you in your use of color. It does not matter if it is an oil painting or a chocolate bonbon, all the same.