You did not mention what you have tried.
The following is from Help and Support:
To change the number of colors displayed on a monitor
1.. Open Display in Control Panel.
2.. On the Settings tab, in the Color quality list, click a different
color setting.
Notes
a.. To open Display, click Start, click Control Panel, and then
double-click Display.
b.. Choose Medium to display over 65,000 colors, High to display over 16
million colors, and Highest to display over 4 billion colors. Choosing more
colors provides better color quality on your screen.
c.. Settings that display a larger number of colors normally require more
computer memory and might affect performance on older computers.
d.. You can switch your color quality temporarily to 256 colors if you
have games or other programs that perform better or must run at that level.
To switch to 256 colors, right-click the game or program (on the desktop or
on the Start menu), and then click Properties. Click the Compatibility tab,
and then select the Run in 256 colors check box. Your display reverts to its
default color quality when you close the program.
e.. Your monitor and video adapter determine the color settings that are
available for your screen.
f.. Changes to the color settings affect all users that log on to the
computer.
g.. Only the recommended color settings are listed. For additional
settings, click the Advanced button on the Settings tab, click the Adapter
tab, and then click List all Modes. Select the resolution, color level, and
refresh rate you want.
h.. If you are using multiple monitors, you can specify the color settings
for each installed monitor. If you click the icon for a secondary monitor,
the Extend my Windows desktop onto this monitor check box must be selected
to change the settings for that monitor.
