The datasheet for the rgb packages that I'm using state average & max ratings, average is 2v for red, and 3.2v for g&b. Max ratings are 2.6v and 4v respectively.
Now we are getting down to what AG was talking about. The datasheet should have a
maximum, typical and
minimum Vf specification. This is the voltage "dropped" across the diode when it is turned on. So for the green and blue string not everyone made will turn on with your 12 volts supply. (you would have to be very unlucky, but it could happen) Or they might be dimmer than you would like because there would be little current flowing through them. What I'm trying to say is that for the blue green ones you should only use two per string.
Now on power. To avoid burning them out to soon you should use the minimum Vf and calculate the resistor to keep the current below the
maximum forward current spec. from the datasheet. Then check again at maximum Vf to make sure you still have a reasonable amount of current.
Right, so string b at 3.2v... 12-9.6= 2.4v, 2.4/.05= 48ohms
0.05x 9.6= 0.48watts. or 0.0025 x 48 = 0.12watts
If you use P=IE it is the voltage across the resistor not the LED that determines the power. So it is .05 X 2.4 or .12 watts.