Try this:

1) Conditional formatting
If cell A2 should change color when its value does not match cell B2

Select the cell (assuming cell A2)
Then....From the Excel main menu:
<format<conditional formatting
Click the dropdown and select:
Formula is: =A2<B2
Click the [formatting] button and set the format
Click the [OK] buttons to finish

2) Conditional summing
C3: =C1+(C2100)*C2
or
C3: =C1+IF(C2100,C2,0)

Does that help?
***********
Regards,
Ron

XL2002, WinXP


"Dom" wrote:

Two questions:

1) I want a cell to change colour to red if it does not match the value of
another cell. I can not seem to work out how this happens.

2) I want two cells to add together but only if one of the has a value of
over 100.
For this i was thinking if would be =IF(c2100, [=SUM(c2+c1)], [c1])
Basically I want c3 to show the value in c1 unless c2 is over 100 in which
case I want it to show the sum of the two.