This is a result of not cooking your mushrooms long enough or cooking them on too low of heat. This will make the mushrooms pale and thus your sauce It is important to make sure that you allow the mushrooms to brown before adding the other ingredients. The mushrooms also need ample room to cook. It is important that you do not overcrowd the pan. Try to cook the mushrooms in a single layer. I find that using a large skillet gives the mushrooms plenty of room to cook without being too crowded. The surface area is usually plenty large.