HP QMS 1660 - Manual (Page 137)

HP-GL Color
Encoding
5-37 Additional Technical Information
Example
Any color can be reproduced by using some combination of CMYK. If
you have defined the following CMYK values for the color brown in
your application, the NTSC equation that follows shows you how the
percentage of grayscale for the color is derived.
CMYK Value
C (96)
M (128)
Y (224)
K (0)
[(96*0.3) + (128*0.59) + (224*0.11) + 0] ч 255 = [(28.80) + (75.52) + (24.64) + 0] ч 255 = 50%
L