They will show colors about 3-4 months old, sometimes you can see the color on the males underside start bluish under his chin and on sides before that age.

I have 5 kids right now that are almost 6 months and the green on the males is coming in nicely.

Males also have a pair of large scales just under the vent area, the slit.

This is what the underside of young male will look like, at a few months old

Young male emerald swift
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PHEve / Eve