Are you trying to apply the Conditional Formatting to the choices presented in the drop-down box?
That is not how Conditional Formatting works. It is applied to the value stored in the cell (or the value returned by a formula in that cell).
A drop-down box presents different options for that cell, but until one is selected, it is NOT the value of that cell. So I don't think you can apply Conditional Formatting to the different options for that cell.