[jacobolus]

In general if you want to avoid any color changes in blobs a few pixels in size, you’ll want to take it easy on the compression, and take the hit of a larger file size in trade.

I suspect that if you give this algorithm twice the file size as a budget, that green color will come back.

[mattpavelle]

I agree, giving more file size may get us our colors back. And after some experimentation I'd like to be able to confirm something a bit abstract like, "Guetzli is good for reducing artifacts by sacrificing color" or some such snippet.

It would definitely have its uses as such. Or maybe it's great all around and I just found one bad example?

[JyrkiAlakuijala]

Guetzli sacrifices some chromaticity information, but tries to keep that in balance with the intensity loss. Guetzli sacrifices colors much less than the commonly used YUV420 mode -- the common default mode in JPEG encoding.