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I did a little test with various screenshots in game vs the sprites I am editing.
The colours you see in the gifs here do not change whatsoever when in game, no matter the lighting conditions. With Umbreon, even when in the bright light of Pokemonamie and the darkness of night it remained the same shade and hue of colours.
There are very slight differences, but I honestly think it might be jpeg compression shifting the colours, like in the case of the top left Umbreon's eyes being darker.