I agree with olenazh that it's a little nit-picky to ask them to repaint the wall just because it's hard to detect dirt on it. If they're storing their RM properly there shouldn't be a lot of product being flung onto the walls. I would be more concerned with having a plain white wall in a processing room where it's common for debris to get everywhere.
It may be more productive for you to think about: How often is the wall (and the RM room in general) cleaned? How can I verify that the wall is indeed clean? A lot of areas in a food production plant get cleaned routinely even if they aren't "visibly dirty". In that sense, a routine cleaning schedule makes the color of the wall less relevant. If an auditor is upset with the design of the wall you can point to your cleaning history and swab results.