In the second picture, white represents fat and black represents lean. Red is background. In the third image, you can see that black is present in 10.0 units and white in 2.5 units. Red is irrelevant. The total for black and red is 12.5 units so the percentage of the second image that is white is 20%.
If you take the cross section of the pork shoulder to be typical, then the visible fat percentage is around 20%. However that is not the primary reason for posting this - the primary reason is to illustrate the technique of measurement, which you can do with a color scanner and Paint Shop Pro.
