C = 1.0 - (R/255)
M = 1.0 - (G/255)
Y = 1.0 - (B/255)
K = MIN(C,M,Y)
C = (C-K)/(1-K).
M = (M-K)/(1-K).
Y = (Y-K)/(1-K).

Unfortunately if you mix BLUE ( CMYK = 1,1,0,0) with YELLOW ( CMYK = 0,0,1,0) you get grey (0.5,0.5,0.5,0) not green ( 1,0, 1 , 0) 

I think the solution is to go to HSL space but be aware of the possibility that you can go either way round the colour wheel.

sheep