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It's more than a light color that decides the turtle's path. So it's "undefinable for now" Amber to orange is safely in their less-effect zone. finding a fixture that fits, with a socket that will take the amber bulbs available is hard. So I was trying to make a turtle-friendly light fixture this is what I found.
Kalvin = NM
This one shows a 4500k at orange. I have never seen a 4500k bulb that didn't look white.