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.