The OS3.9 drivers (also in OS3.1) will allow you to use a regular PC VGA monitor with an Amiga in special DoubleScan non-flickery modes, but I'm afraid they will not expand the range of colours.
In fact, I think most of those Double Scan modes (certainly Productivity Multiscan mode) will actually reduce the number of on-screen colours to four.
Again, though, these modes are limited (only Workbench & some applications - no games).