for r for g for b

l = []

for r in range(256):
    for g in range(256):
        for b in range(256):
            if x < w:
                pix[x, y] = (r, g, b)
                x += 1
            else:
                #Don't use this color yet. Creates the diagonal.
                l.append((r, g, b))
                x = 0   
                y += 1

#this is the last row of pixels                
for i in l:
    pix[x, y] = i
    x += 1