Use the "Conditional Formatting" option on the "Format" Menu.
The simplest method is to compare the cell value to a specific number. For instance, you could apply a format when ever a number is less than zero, or when it is between -3 and +3.
The next option is to campare to another cell. For example, you could calculate the upper control limit and then compare a series of cells to that cell.
The final option (I I just recently discovered) is to create a formula as a condition. When the conditional formatting box is open, look at the left most drop-down menu and choose "Formula Is". For example, if you select a range (starting at cell B4) and type the formula
it will test if something is 30 days past due. If you add a second condition like
it can test if it is coming due in the next week.
After you set the condition, you can have the cell color change by selecting "Format", the "Borders". Perhaps a month over due is red, slightly overdue is orange, and due in the next week is yellow.
I could attach a simple example if it would help...