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Not just PNG and SVG, also JPG.
https://commons.wikimedia.org/wiki/File:BlueCyanGreen_PNG_screenshot.png
https://commons.wikimedia.org/wiki/File:BlueCyanGreen.jpg
From the following file I get the impression, that the problem is negligible for TIF files:
https://commons.wikimedia.org/wiki/File:IUSM_logo-2.tif

Sounds a bit similar to T141739

Is this issue still present? After version upgrades it appears to me that thumbs now match the original colours

In T329336#8826122, @hnowlan wrote:

Is this issue still present? After version upgrades it appears to me that thumbs now match the original colours

I just uploaded File:Blue 00f and green 080.png. The thumbnail colors are still wrong, see e.g. on the category page.

This also affects shades of red. Here is an example where pale #eb4e4e is rendered as bright #ff1d3e:
https://commons.wikimedia.org/wiki/File:Family_1110_1111_1110_1010_(Zhe_8329).svg

This can probably be closed. I just realized, that my browsers show some thumbnails with wrong colors.
Maybe I was wrong about this in February. Or maybe the problem existed then, and was fixed with the mentioned update.