Is there any evidence for the use of more than one color in early medieval clothing cut along rectangular patterns? I know the rectangular cut itself is meant to create a whole garment from a single roll of cloth with the least possible waste, but I can't help thinking about some simple swapping--say, in making two T-tunics for the same person, identical in every respect except for their color, is there any historical precedent that would allow me to swap the arms of the tunics so that the red tunic now has green arms and the green tunic has red ones? I've never seen any such thing in early medieval illustrations yet. My knowledge of them is far from thorough, however, so I still can't help wondering about whether there might be some evidence for this color-swapping method.

As a general rule, no.
It becomes fashionable in the 14th century but is not common before that.
Lest, of course, you be a yester, musician or other amusing character.