HP Vectra Reference Manual page 111

Color Register
The color register is a 6-bit write only register, that applies to the
alphanumeric mode.
Bit Data Definition
0 0 Off, blue
1
On, blue
0 Off, green
1 On, green
2 0 Off, red
1
On, red
3 0 Off, intensify
1
On, intensify
4
Alternate set select
5
Pallet select
~
The use of bits 0, 1, 2, and 3 depends on the mode of operation
selected. Defined as follows:
Alphanumeric: selects the screen border color. *
320x2 graphics: selects the screen background color.
640x1 graphics: selects the foreground color, background black.
Bit
4 when set to 1 intensifies the 320 x 2 colors to their
alternates.
*Not implemented for 25kHz monitor.
94 Multi-Mode Video Adapter Card