I wrote a patch for the "props" lexer which makes it possible to highlight the left and the right side of an assignment in two different colours. The patch requires Scintilla/SciTE 1.67. First download the patch from HERE and then apply it by running patch -p0 < scite167_props_patch.diff.

Update: This patch will be included in the next official Scintilla release. The latest source at the Scintilla CVS already includes this feature.