Hi Greg,
Can only be done in a macro and very inefficiently, because
it would have to be a volatile macro. If you are willing to do
your own calculate (F9) or trigger a macro when the sheet
is brought up, or you double click from a sheet then you can
use non volatile macros. In other words relying on color
alone is not a good idea and there are many forms of color
blindness.
You can probably get what you want through Conditional
Formatting if you can use a formula to determine what
color you want. Conditional Formatting must not refer to another
sheet, so you might be able to use a helper column.
For the macro solution see
Functions For Cell Colors
http://www.cpearson.com/excel/colors.htm
Many answers to questions on color can be found at
http://www.mvps.org/dmcritchie/excel/colors.htm
and many answers to Conditional Formatting can be found at
http://www.mvps.org/dmcritchie/excel/condfmt.htm