You can't specify a tray.
Limited fax memory, long weekend and the tray runs out, you lose your faxes and get upset with Xerox ;-)
But...
You could program tray 4 to be Blue or Recycled paper, then go to Machine Status > Tools > Service Settings > Fax Service > Incoming Fax Defaults > Paper Settings > Manual and set it to use such stock.
The caveat is that it will not be able to auto tray switch since nothing else would be loaded this way, and print/copy jobs won't be able to use or switch to it for the same reason unless defined in the job.
<You getting upset at Xerox SirBubby is just an example as to the thought process I assume the engineers went through, I'm sure you are a fine person who would do no such thing ;-) >