How to flag a cell a colour based on measurements entered elsewhere?

I have a spreadsheet which calculates cost of lots of different types of materials based on the size entered.

there is a length cell (B5)
and a width cell (B6)

these are there for the user to enter the size into in mm.

the table below that has lots of types of materials with rows of prices which change based on the size entered and each column represents what the material will be applied to.
(I have attached a screenshot of it)

for an example i will use row 13 which represents Vinyl 610mm
there is column for media only, and the vinyl applied to various types of board 3mm/5mm/10mm etc, the formula in each cell calculates the cost based not he measurements and base costs worked out on a separate tab of the spreadsheet.

how can i additionally tell specific cells to highlight a colour if the size entered is larger than the media to warn the user?
For example cell B13 is the Vinyl, and shows the cost for the vinyl only at the size entered, but the maximum width for this is 610mm so i want it to highlight red to warn the user that the size entered is not possible.