I have attached a monitor to the DB-15 video port on the back of my Power Macintosh 7100/80. Why can't I display less than 256 colors?

Monitors attached to the High Performance Video (HPV) card on Power Macintosh 7100 and 8100 series computers cannot display less than 256 colors.

To display less than 256 colors, you must connect your monitor to the HDI-45 video port. Using this port allows your monitor to access the built-in DRAM to display images. A video adapter cable (Part #M2681LL/A)* is needed to connect a monitor with a DB-15 connector to the HDI-45 video port.

The Power Macintosh AV Card also has the ability to display less than 256 colors.