Actually, I was a bit busy writing that between cases and did not have a chance to elaborate. 
It is for a Charlieplexed switch matrix that there is the requirement that three times the diode voltage drop exceeds the driving voltage. For this reason, you need to use green(, blue or white) LEDs as the diodes in such a matrix.
For a Charlieplexed LED matrix, you could get phantoms if using red LEDs and a LED that is supposed to be illuminated, is faulty or missing so that it does not limit the voltage. 