Well, the issue seems to be on the SVG implementation :-)
For text containing only spaces, even with some background color, SVG rendering do not display anything :-(
A possible workaround could be to use "IDEOGRAPHIC SPACE" (see http://jkorpela.fi/chars/spaces.html )
Bob -> Alice : hello
|= |= Type |
|<back:red><color:red><U+3000></color></back> | Abcdef |
|<back:blue><color:blue><U+3000></color></back> | Uvwxyz |
Note that we *could* implement the workaround in PlantUML code (that is changing pure "space" text to "ideographic space" for SVG output), but that would be weird, so if possible, we'd like better that you use the workaround.
Any though ?