Hi Banqueteer,
What type of shrooms are you using?
How are you cooking them?
Like Suzanne said, Mushrooms with naturally turn after cooking.
If you add your shrooms to a real hot pan you will get a nice outer caramel color that gives the shrooms a wonderful flavor and aroma.
If your pan is not hot enough the liquid from the shrooms will leach out and basiclly boil in there own juices.
Also don't salt your mushrooms right away, wait until they develope a nice color. Salting to soon will also draw out the water.
Also keep in mind that most "true" wild mushrooms don't really discolor that much or at all. Take Morels or chanterells as an example. Black trumpets and lobster shrooms, hen of the woods Ect all keep there basic color when cooked.
Just remeber that mushrooms really like HOT pans and hate to swim in there own juices.
Exotic or culitvated mushrooms like plurottes (oyster) crimini, portabella's and shittakes do tend to discolor (not so much plurottes but i'm not a fan of them)
So if price is an issue you can blend some types of culitaved shrooms with the wild guy's.