color coded cells and conditional format in excel 2003

Hi, I work with invoices from my suppliers to make the payments and I put in
excel sheet with invoice number and supplier like

A2: supplier 1 - inv. number
A3: supplier 1 - inv. number
A4: supplier 2 - inv. number
A5: supplier 3 - inv. number

What I'm trying to made is shade the rows from supplier 1 as white, supplier
2 as gray 25%, supplier 3 as white or any other color. I have many suppliers
and vary from month to month. The conditionnal format doesn't find a way to
solution. I have tried to put a condition IF(A2A1) gray else white, but in
the next row the A3=A2 it will paint as white instead gray. My objective is
distinguish the suppliers at a glance. Thanks
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With best regards,
Looksmart5000