[HTML table version removed; see GIF version]
the left panel was made with GIMP and Online Image Editor; the right was made with GIMP (after I figured out how to use it)
At some point I converted these backgrounds to "indexed color" (don't know how) -- this meant I couldn't change the color using the bucket fill (don't know why).
I learned from "help" that I could change the image "mode" from indexed to RGB.
That enabled me to add the background colors.