Continue Cooking Until Desired Color Is AchievedHow long you cook the roux depends on what you're using it for. A béchamel sauce calls for a white roux, so you'll only want to cook it for a few minutes, until the raw flour taste is gone but the roux is still a pale yellow.
A blond roux, used in white velouté sauces, needs to be a bit darker, so it's cooked a minute or two longer.
A brown roux, used in brown sauces, is the darkest roux, and it's cooked for the longest amount of time. For that reason, you should cook it over a lower heat so that you don't burn it. You can even brown the flour in the oven before adding it to the butter.