unions, intersections or array constants

[MyVeryOwnSelf]

This ought to avoid false hits:
=AND(LEN($AK$2)=3,SEARCH($AK$2&".","JAN.MAR.MAY.JUL.SEP.NOV."))

The extra "." is to avoid false hits like "ARM".

 

[Loadmaster]

T. Valko, to answer your questions on if these are user generated formulas is
you gave me the two array formulas. The one in A3 is:

=LOOKUP(9.999999999999E+307,INDEX(D2:AH13,0,MAX(IF(D2:AH13<>"",COLUMN(D2:AH13)-COLUMN(D2)+1))))

and, the one in AK2 is:

=INDEX(C2:C13,MATCH(1E+100,INDEX(D2:AJ13,0,MAX(IF(D2:AJ13<>"",COLUMN(D2:AJ13)-COLUMN(D2)+1)))))

 

[T. Valko]

Ok, that helps to settle all of our "yeah, but if..." scenarios.

This went from a conditional formatting question to a data validation
contest.

 

[Harlan Grove]

If the cell contains a number from 1 to 12 one or the other formats is
applied.

....

Good point.

blue: =IF(ISTEXT($AK$2),MOD(MONTH($AK$2&"-1"),2)=1)

green: =IF(ISTEXT($AK$2),MOD(MONTH($AK$2&"-1"),2)=0)

These allow whitespace in AK2. If that's not OK, then change the
second cell
references to SUBSTITUTE($AK$2," ","%").