Digital BW, The Print

Another option I thought about was writing a small program to convert 
grayscale images to false color for quad-tone printing.  The inks I 
got were 25% black for cyan, 50% for yellow, 75% for magenta, and 
100% for black.  Here's one simpleminded approach:

1. Convert all pixels with values between 0 and 25% to cyan.
2. Convert all values between 25 and 50 to a mixture of cyan and 
yellow.
3. Convert all values between 50 and 75 to a mixture of yellow and 
magenta.
4. Convert all values between 75 and 100 to a mixture of magenta and 
gray/black.

In general, can anybody describe an appropriate conversion function 
from 8-bit grayscale value to RGB or CYMK value?

Mark