Roux, when completed, comes in varying shades of off-white to the much darker brown shades. There are four names that are generally associated with these varying shades: white, blond, brown and dark. The color of a roux will depend on the temperature and time that you take to cook the flour. Flour that is cooked over a low temperature for a long period of time will be a lighter color; a white or blond roux. Flour cooked on a high heat quickly will be darker (brown or dark roux), have a stronger flavor, and will not be as powerful a thickener as a lighter roux. None of the types of roux are necessarily better or worse than any other, the color you choose will depend on the flavor and color you would like your roux to lend to your sauce or soup.
To make a basic roux start by measuring, by weight, the amounts of fat and flour desired. It's a one to one ratio (e.g. 4 ounces of fat and 4 ounces of flour equal 8 ounces of roux). Butter is the most commonly used form of fat. Melt the butter over medium heat, careful not to start browning it, then slowly add the flour to the butter, whisking constantly. Within 2 to 3 minutes the roux will have a consistency of a cake frosting.
Both fat and flour have various levels of moisture content and the possible choices of fat and flour that can be used are many. For example, butter contains a small amount of water. However, bacon fat, which has been perfectly rendered, contains no water. Within these variables there are varying levels of consistencies. If your roux is too thin, add a small amount of flour. If it is too thick, add a small amount of fat to thin it out. The consistency in the beginning of creating a roux is important, as it could break over time. "Breaking a roux" is the ultimate roux no-no. It happens when the fat separates from the flour granules, as opposed to being absorbed by them making a smooth and consistent paste-like texture.
When cooking with a roux, be sure to add only cool liquids or ingredients to a hot roux, or vice versa. As you incorporate the liquid into your roux, be sure to whisk frequently and add the ingredients slowly. Then heat the entire mixture until it comes to a simmer. This process will keep the roux from creating lumps. Once all of the liquid has been added to the roux, cook the sauce or soup for at least 20 minutes, otherwise your finished product may have a granular or gummy texture.