For quickness, I've just used Walli's Noble One tree.
The image you'd normally use for the leaves is on the left.
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But when you try to apply a new colour(blue) for diffuse, it doesn't work very well, giving a strange purpley colour because it's mixing the already coloured image with my new blue.
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If I desaturate the image, however(right-hand leaf image), I can use that greyscale version and apply it by the same blue colour I used above, which will now work correctly.
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