Diesel fuel comes in a variety of colors, but in its natural state it ranges from completely colorless to amber. The amber color is primarily due to sulphur content and oxidation. Diesel is also often dyed red or blue in the United States to indicate that it is tax-free.
Undyed diesel darkens due to oxidation if not stored properly. Highly refined diesel contains less sulphur and is consequently lighter in color, even trending toward light pink, orange or green in appearance. These colors have no bearing on the effectiveness of the fuel.
The light hues present in undyed diesel are less conspicuous than the red and blue colored diesel authorized for use in certain vehicles. Both red and blue diesel are federally mandated as tax-free and are dyed to prevent use in unauthorized vehicles.