1. A method of comparing first and second digital color images having registered pixels, each pixel having data for N colors, the first digital color image having an appearance of a physical activity or object at a first time, the second digital color image having an appearance of the physical activity or object at a second time, the method comprising: forming a composite image having pixels in a 2N-dimensional color model by defining color data for each pixel in the composite image as a concatenation of the color data for a pixel in the first digital color image with the color data for a registered pixel in the second digital color image; determining a set of principal components of the composite image using a principal component analysis, the set of principal components defining a coordinate system in the 2N-dimensional color model; selecting a subset of principal components in the coordinate system as those principle components that each represent a difference, rather than a correlation, between the first digital color image and the second digital color image; and for each pixel in the composite image, computing a normalized pixel change value as a function of the coordinates of the pixel color data along each principal component in the subset, the normalized pixel change values collectively defining a change image having a one-dimensional color model, the computed normalized pixel change value for each pixel measuring a change in the physical activity or object between the first time and the second time.