You can tag quotes with a colour (just like you can with jobs on the shipping calendar). The colour you choose and the reason for the tag is entirely up to you.

Tick the boxes next to the quotes/invoices you want to tag (1). From the top green dropdown, select the colour you want to tag (2). The background of the central column will be tagged with the colour selected (3).

If you want to sort the quotes by tagged colours, just click the sort arrow at the top of the column. The tags will sort based on the number assigned to each. You can click the sorting arrow to switch between ascending and descending.

Setting the tags to 0 will effectively untag them, as this is the same colour as the page background and what each quote is set to by default.