I need to using format conditioning to change a cell value. I cannot use a IF Statement.

I known how to change the background colour of cell A3 to green, when Cell A1 = "F".

My Problem:
I want the value of Cell A3 to be change to "X", by two methods:
1) The user can manually to place an "X" in A3, or,
2) The program to automatically change the value of A3 to "X", when either the background colour of cell A3 is Green, or Cell A1="F"

I know how to do Option 2 using the IF Statement, but Option 1 will overwrite and remove the IF statement.

Can anyone help?