Alerts? Who said anything about alerts?
You can import both the Automatic and Manual styles into the customer record. Imports can be automatically run on startup. You could also create a note about the change of status and import that at the same time.
So yes, this can be almost completely automated with a GridView View.
You could also implement this with 3 Filters with different Priorities.
Name Priority Style
YTD Sales > 0 OR 100 Green
LY Sales > 0
YTD Sales > 10,000 OR 50 Yellow
LY Sales > 10,000
YTD Sales > 20,000 OR 1 Blue
LY Sales > 20,000
The color would be updated each time the DIC was run, either through DataCare or within the application. Editing a customer would also update the color when the record was saved.
Of course, the Filter approach references the YTD Invoices within the customer record, so it is a little more coarse an indicator than you can accomplish with GridView.