To turn this into a video, as per the Facebook advice, I used this:
ffmpeg -loop 1 -i rainbow.png -c:v libx265 -x265-params lossless -pix_fmt
yuvj420p -t 5 rainbow.mp4
and then extracted a frame like this:
ffmpeg -ss 00:00:02 -i rainbow.mp4 -vframes 1 rainbow-extract.png
and then took screenshots for comparison from SMPlayer, and YouTube (both
Firefox and Chromium).
Here is an annotated comparison (attached as rainbow-comparisons.png):
https://drive.google.com/open?id=12wgVPPzdp2Qi-tEMBoVg-3E8CIZW6Jvh The screenshots all look very similar. Something I find odd is that the
extracted frame is visibly slightly different to the source picture.