Serial Number History Showing in Red

Introduced in version 9.100, when the Serial number history is selected, it will show the serial number in red.  This is to indicate that the product has been entered to a job and not yet invoiced.

If the Sale Info button is selected for the red serial number, a prompt will appear

If a serial number is assigned to a Pending invoice, this will not show in the Serial History screen.

If the serial numbers are showing in red 

and when you select Sale Info, a prompt relates to a Pending Invoice

When the Invoice is opened in Read Only Mode, and a check it shows that the invoice is posted.

This is because the product is ticked to allow multiple serial numbers.