Save you some time:
VLMx1300 (red, Vishay, LED), If = 20ma @ Vfwd = 1.95v; If = 30ma @ Vfwd = 2.0v (delta Vfwd = 50mv). delta Rd = 50mv / 10ma = 5ohm.
higher Rd for organge (50mv / 5ma) and lower for green.
None of this factors in the output resistance of the avr's (30 - 50ohm for example).