Highlight order issues within a form

I have created a form (frmProductionSchedule) that contains a combo box (ProductCode).  Based on the entry of that combo box and AllergenCheck field is populated.  This field contains combinations of the letters S, W, M and E (representing various allergens).  I would like record two's AllergenCheck results to be compared to record one's results.  If record two contains all the same letters as record one (and potentially more letters), than no highlighting, but if record two is missing a letter that is contained in record one than I would like it highlighted.  The record in question would always be compared to the one directly above it.  In the scenario below, I would like AllergenCheck field for ProductCode 4 and 9 to be highlighted.  Can you help?

ProductCode;AllergenCheck
1;      S
2;      S
3;      S
4;      EMW
5;      S
6;      S
7;      S
8;      SEMW
9;      SEM