Grumpy_Mike:
but maybe there is a better way?
Yes you have the LEDs, connect them up with something like a 330R resistor and actually measure the forward voltage then you can do the math correctly to get your current.
I hooked up a 220 ohm on the red, and a 100 ohm on the green. Im using 3.3v from an arduino nano, and with the red LED I get 2 volts, with the green I get 2.7, the green seems pretty bright, but the red doesnt seem very bright.
Calculations Red (3.3-2)/220 = 0.005909091 (or about 6ma)
Green (3.3-2.7)/100 = 0.006 (or 6ma)
Green (3.3-2.77)/75 =0.007066667
Green (3.3-2.88)/50 =0.0084
Red (3.3-2.2)/50 = 0.022 (hey, getting close)
Red (3.3-2.1)/75 = 0.016
Sadly 50 ohms is the smallest resistor I can find.
From that can I calculate what voltage I need for 20ma?