Note that there's a compositing bug in Chrome that may cause semi-opaque colors to be rendered wrong: https://observablehq.com/@mootari/chrome-transparency-bug

Interesting discovery! Do you suppose I could take better measurements than screenshots?

I'm not sure that you can, tbh. The way I did it after I was to use macOS' "Digital Color Meter" in combination with a few reference colors for which I knew the expected channel values. In general you should be safe if you don't use Chrome (with hardware acceleration) or have an sRGB color profile set for your display. I discovered the bug while working on a similar notebook that turns opaque colors into their transparent counterparts at arbitrary opacity (within the feasible limits). Thought I was going crazy until I found out the reason for the mismatches.