Thanks ALL, it is a patient process..
Androo, yes, the colour achieved isn't via "the half way point"....but via variations of colours...the green cherry I've got is derived from a yellow having a green variation, a choc having a green variation and a blue velvet having a green variation....through several generations, and via a selectively picking the best out of the colony, you attempt to improve the coloration through the selective process...... I have 3-4 that have the same green intensity, and approx. 7-8 with a lighter green tinge.... these are all achieved from 5-6 generations, with 100's of culls etc....
In terms of purple, yes, the variation exist in the cherry family .... just a slow & difficult process.... I was working on selectively breeding them, BUT was slack, and it all turned to the wild form, as I didn't cull and selectively breed them as well as I should have :-(