Right-click a blank part of the desktop and select Properties then Settings.
Select an item from the Colours list that matches the setting required by
whatever the procedure is that you are running.

Should 256 not be an available choice, you will probably need to install the
right display driver.  My standard blurb on this issue:

When the correct screen resolution (screen area) or the right number of
colors are not available choices in display properties, settings tab, then
it is a good bet you do not have the right driver installed for your display
adapter.  The display adapter in your PC is either a video card or a display
chipset on the motherboard.

If you received a CD or floppy disks with drivers along with your PC, see if
they contain a display adapter driver and install it using: control panel,
system, device manager, display adapter.

If you know what display adapter your PC has but you have no disks, use
www.google.com to track down the correct driver on the internet.

If you do not know what display adapter your PC uses, you can go to a MS-DOS
prompt and type:  Debug       and hit enter.  The screen will display a
flashing prompt next to a - sign.  Type:  DC000:35 (DC000:50 may also work)
and hit enter.  The name and possibly model of your display adapter should
appear on the right hand side of the screen.  To quit Debug, type Q and hit
enter.  If Debug is not helpful, you can try this program:  Advanced PCI
Info, available at:  http://www.upsystems.com.ua/support/alexmina/   If all
else fails, you will have to remove the computer case.  Look at where the
monitor plugs into the back of the case and then check that location inside
the case.  If there is a card there, you obviously have a video card and if
not, you have on board graphics.  Write down any information displayed on
either the card or the chipset on the motherboard.  Then use www.google.com
to search for the information.  Do not include all the data you found in one
search message--search on each piece individually.  If you include all the
information at once, you might not get a hit.

I have found that a video card will generally have to be removed in order to
see the information.  Be careful of static electricity as it can fry
components.  Before touching anything in the computer case (the cord is
unplugged, right?), ground yourself to the case by touching it.  Don't work
on carpeting since shuffling your feet on it can generate static electricity
after you grounded yourself.  Remove the one screw that holds the card in
place and using a rocking motion (left to right and back) and pull the card
straight out.

Once you have the right driver, install it through control panel, system,
device manager or through control panel, add new hardware, as appropriate.