Excel: Find duplicates among 2 colums

Question

Giving an excel file with 2 columns:

A: 5000 numbers (some repeated) B: 525 unique numbers

I want to know if for each number in A there is a instance of that number in B.

Meaning: for A1, is this value in column B?, if so highlight cell in green, otherwise, in RED.

I actually got the duplicates by using in column C this:

=COUNTIF($B$2:$B$525,A2)

This gives me 0 as False and 1 as TRUE, so far so good!

Now what I want to know is how to build a conditional formatting .

1. I go to Edit Formatting Rule
2. Use a formula to determine wich cells to format

3. RULE:

   =COUNTIF(B2:B9999,A15)
   

   A15 because cell A15 is true, A2 to A14 is false.

4. Applies to: A2:A9999

However this highlights random cells.

So I think I did the hardest part but I am stuck in what seems to be the easiest.

Any guidance?