well, if you add a section to the group footer, and set your summarized value to a shared (or global) variable, then when you actually display the values, you can tap into your shared variables and use them to change the background color.
another way of saying the same thing. find the values that you want to compare. store them in shared variables in a section above the displayed value, then access the variables to change the color.
example
gf1a:
shared numbervar col1 := {table.field1};
shared numbervar col2 := {table.field2};
"" //will suppress any display of the values (though you can also suppress the section
gf1b, {table.field1}, format object, font (i think) color, check the background(ithink)
shared numbervar col1;
shared numbervar col2;
if col1 <> col2 then
crRed
else
crWhite;
HTH