I'm not sure what type of snake you mean. Are you talking about those snows that tend to have greenish colored blotches?
If so, if I'm not mistaken, those are pretty rare.
If you just mean snows that have lime-green outlines to their saddles, this is more common. I know IcedGoddess and I both have snows that look like this. I'm sure a lot of other people do as well.
As far as when you start to see this in snows, I can't recall when I first noticed it, but it was probably at around 1 year old.
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alstiver@hotmail.com
1.0 '01 hypo snow
0.1 '02 ghost (pastel)
1.1 '02 bloodred