When a match from /mt is automatically inserted into the segment because the match is superior to the threshold set in the Editor, the segment background is not changed to the color associated to /tm matches.
It is only when the match is manually inserted that the colored background is set.
fix in https://github.com/omegat-org/omegat/pull/496
I think this and https://sourceforge.net/p/omegat/bugs/983/ are duplicate. If so, please close the other bug report as well :)
Sorry, I should have noticed it.