colour changing cells

Paskylou 

It's definitely possible. What I've attached does it. I deviated from your description just a bit, so that rather than adding a date when the item comes in, you simply select a word "arrived" from a drop down. That word turns the cell green. In the absence of that, whether the word is "waiting" or G3 is simply blank, there's a "helper cell" that tracks the difference between today's date and the date scheduled for the customer to pick up. That helper cell can be off to the side, and even in a hidden column. The shorter the period, and you can adjust these in the Conditional Formatting dialog box, the redder it gets.

Note: there may be more streamlined ways to do this in conditional formatting, and I'd love to see what others come up with.