Quote Originally Posted by starguy
If cell A1 contain the optoin of X or blank
Then apply Custom Data Validation in A2 as

=AND(A1="X",OR(A2=1,A2=2,A2=3))
remember to uncheck the Ignore blank option.

And in cell A3 use this Custom formula

=AND(A1="X",OR(A3="red",A3="blue",A3="green",A3="yellow"))
remember to uncheck the Ignore blank option.

----edit-----
formulas wrapped
Thank you for your advice. In the end I used a bit of both.

In an empty column (which I later hid) I placed the list of entries for A1 (i.e. " " and "X"), a list of entries for A2 (1, 2, 3), and a list of entries for A3 (red, blue, green, yellow).

I then created a List Validation for each cell (A1, A2, A3), referencing their respective lists.

For Cells A2 and A3 though, for the List source I entered an if statement such as:
=IF(AND(A1="X", A4=""), $Y$54:$Y$57, "")
- where $Y$54:$Y$57 indicated where the list was located, and where no entry was permitted in A4 for a value to be selected in A2 or A3.

Thank you once again for your advice. It was much appreciated.