1. A method implemented at least in part by a computer system, the method for determining a color classification of a digital image comprising pixels each having at least three color values, and the method comprising the steps of: converting the color values of at least some of the pixels to a lightness value, a first color difference value, and a second color difference value, the at least some of the pixels being selected pixels; assigning each of the selected pixels to one of a plurality of lightness level groups based at least upon the selected pixel's lightness value; using the computer system to calculate, for at least some of the lightness level groups (“selected LLGs”), a first mean of first color difference values and a second mean of second color difference values of corresponding selected pixels; using the computer system to calculate, for each of the selected LLGs, a mean color hue angle value from the first mean of the first color difference value and the second mean of the second color difference value; and determining whether the digital image is monochromatic based at least upon (a) an analysis of a distribution of the first and second color difference values in each of the selected LLGs with respect to the calculated means, or derivatives thereof, and (b) an analysis of the distribution of mean color hue angle values for all the selected LLGs, or derivatives thereof.