Comment by phantomics

The pixels aren't singular values, the image is a height×width×3 array of 8-bit integers. The third dimension is the three RGB values for each pixel. The bulk of the string, the "(3⍴2*8)⊥3 1 2⍉⍵" part, converts the array into a height×width matrix of 24-bit integers representing the colors. Once you have a matrix M, "⍴∪,M" is all the code needed to count the unique values.