You can use conditional formatting for this. You might also consider
unlocking the cells which have "adjustable" numbers in and then
protecting the worksheet, so that your formulae do not accidentally get
over-written. I usually use a bright yellow background as well to
indicate to users where they can enter or change data (i.e. the
unlocked cells), and so the normal white background indicates that they
cannot change anything in that area.

Hope this helps.

Pete