Change of Cell color

I want to change the color of a cell if the value of the cell do not satisfy a condition,e.g.
if(A4-A5)>A2 then the color of cell 6 will change to red.

In Excel, there is no such thing as a "cell 6". Did you mean cell A6?

Also, "if(A4-A5)>A2" is not a valid formula. I assume that you meant if A4-A5>A2, then change the fill color of A6 to red.

If that is what you are looking for, select A6 > Conditional Formatting > New Rule > Use a formula

=A4-A5>A2

Format: Fill red > OK > OK