Roux is a combination of flour and fat that is slow-cooked to varying degrees and used as a thickener in dishes such as soups/stews and sauces. The longer roux is cooked the darker it becomes and the more flavor that develops. Roux ranges from white to dark brown, all serving a different purpose culinarily. For more information about roux and to see photographic examples of the varying colors, check out this guide to roux by Serious Eats.