Beer color is measured using the Standard Research Method (SRM) or degrees Lovibond (°L). These scales are essentially the same and are useful in describing the color of beer. You can use this color swatch to estimate the beers color in SRM. The Beer Tasting and Hop Appreciation Kit comes with a set of 12 different colors for you compare your beer to.
For best results hold you beer up to a sheet of white paper in a well lit room or with a bright light shining on the beer. This will show the true color of the beer and prevent background colors from interfering with your view.
These are estimates and to get a completely accurate measurement you would need a tintometer but this scale is close enough to gain an appreciation for your beer's color.