I am trying to use conditional formatting for this problem:

In Column A I have cells which allow you to choose from a list of codes,
from A0001 to A1000 (using a drop down list/validation)

In Column B I have another list of codes (for example, A-Z) (using a drop
down list/validation)

If I choose a code from A0900 to A1000 in cell A1 AND there is nothing
chosen in Column B (cell B1), the codes A0900 to A1000 should turn RED.

If I choose a code from A0900 to A1000 and then choose a code in Column B
(from the A-Z), the codes A0900 to A1000 should return to normal.

(IE The conditional formatting alerts you to the fact that codes A900-A1000
need a complimentary code)

Any suggestions?

Many thanks!