Column G now has conditional formatting, the cells go red if they anywhere contain the corresponding value from A, modify the formula for whatever result you need.
Sorry David but what is the condition you are looking for? Cell G3 contains 2 which is on the list in column A, and you marked that black, Cell G4 contains 2 as well, but you marked it red?
Are you trying to count how many values match, or what is the logic?
I prefer this method: =COUNTIF($A$3:$A$19,ROWS($G$3:$G3))
Because if you insert rows or delete Column A the formula you propose doesn't work. If you take care about the cell you start, you avoid this problem. Blessings!