Update on the color issue with gamma-correct blending in #foot
It seems the biggest issue was that foot used a different transfer function than the compositors. After fixing that, the issue appears to be gone, mostly.
The lack of precision (i.e. the fact that we're only using 8- or 10-bit color channels) does mean some colors will be slightly off. But it shouldn't be noticeable, unless you check with a color picker.
That's at least the working theory, and for now, gamma-correct blending will continue to be enabled by default, when there's compositor support for it.