How to measure and power an array of unknown RGB LEDs?
You've got it covered pretty well. Just  a couple of very minor points.

It appears by the LED's not having dimmer capability you want white light.  The low power LEDs do not mix very well.  You should use LEDs like the Lumiled C line or CZ line which are designed to have good color mixing. White comes in many varieties of CCT and CRI.  CCT determines the Kelvin Temperature (warm, netural, cool) and CRI is how close they match natural sunlight. White LEDs are less expensive and are specified in CCT and CRI. 

When selecting LEDs you may want to consider the Photopic Luminous Efficacy of the Relative Sensitivity Curve for the C.I.E. Standard Observer.
According to this curve when compared to a 555nm green with a radiometric intensity of 100, a 450nm blue LED would be perceived as 3.8 and a 660nm red would be 6.1.  If you are using a 470nm blue they are usually specified in Luminous units (cd or lm) rather than the 450nm deep blue which is specified in radiometric watts.  Same applies to 660nm red (deep red) vs. 625nm red-orange (commonly called red)

There is usually a big difference in the specified luminous and radiometric intensities between R, G, and B LEDs.  If the LED's intensities are specified in cd or lm you may want to adjust the resistors to match the actual luminous intensities.  Matching radiometric LEDs also requires the calculating Photopic Luminous Efficacy.

Links:  Photopic Luminous Efficacy
Irradiance Conversion Factors

A 3.3V supply would be better than 30% more efficient.  

It is best to use the actual measured Vf to calculate the resistor values.

I get slightly different values for the resistors. 

Red 18.75Ω (actual 18.7 or 19.1)
Green 19.55Ω (actual 19.1 or 19.6)
Blue 20Ω

BTW: You said "19 ohm resistor (R1) to bring the voltage down to 2.0v." the resistors limit the current.  The LED's forward voltage (Vf) characteristics determines the voltage.