It's possible that a newer Mesa has fixed this problem already. Because a similar setup (LLVMpipe software rendering in Xmir) on desktop has no discolouration:
Working: xenial with Mesa 11.2.0 (desktop)
Broken: vivid with Mesa 10.5.9 (Ubuntu Phone stable channel)
P.S. If there's any way to tell LibreOffice to not use OpenGL, that might also help.
It's possible that a newer Mesa has fixed this problem already. Because a similar setup (LLVMpipe software rendering in Xmir) on desktop has no discolouration:
Working: xenial with Mesa 11.2.0 (desktop)
Broken: vivid with Mesa 10.5.9 (Ubuntu Phone stable channel)
P.S. If there's any way to tell LibreOffice to not use OpenGL, that might also help.