Conditional formatting a UDF in Sales Oder

We are having some of our vendors put stipulation on our sales saying certain customers can't purchase brands products. We have put memos in customer accounts so when someone takes an order the memo pops up first saying what they can't or can purchase from us. This is a crazy question but is there a way to do  a conditional formatting on a UDF in Sales Order that pops up saying that they can't purchase that item when they type into a sales order? If customer memo doesn't say approved for a Brand and that UDF Brand is on a sales order. Or do you know of a better way to tackle it? I know MAS has a way to do this but it doesn't work for our web orders.