VDU 23,25,n|
VDU 23,25,s+&80,s+1,r1,g1,b1,r2,g2,b2
 Where n is the number of colours (1,2,3,4 => 2,4,8,16), and s is the start logical colour number, fading from r1b1g1 to r2g2b2 in n steps.