Are you certain that the yellow and the green specimens are the same?
I noticed several differences on the green one:
- Wing spots
- Different shaped wings
- Brown head
- Different shaped antennae tip.
You appear to be suggesting that the yellow butterflies appeared green on the ivy, but they actually appear to be difference species.
(Maybe I have misunderstood the point you were making...)
If I were you
And I wish that I were you
All the things I'd do
To make myself turn blue