Yeah... You probably don't need an Arduino... Just something to boost/repeat the 3 PWM values. That's assuming there is some dimming. If the 3 colors are only turned on/off there's no PWM, but the circuit would work the same.
I have already successfully attached wires to the master RGB LED in question and can successfully read the voltages across the red, green, and blue LEDs with my multimeter.
Is that an analog meter? An analog meter will usually average the PWM (because of the mechanical inertia of the movement) but digital meters usually are not reliable with PWM or any kind of pulse.
If you are measuring voltage directly across the LED it will be less than 5V. More than 5V can damage the Arduino, so let us know if you're getting more than 5V.
See the [u]Analog Read Serial Example[/u]. With the Arduino, you'd need a [u]low-pass RC filter[/u] for each color to measure PWM. (A ~10Hz filter should be fast-enough to respond to color changes but slow-enough to filter the PWM.) Note that the Arduino reads voltage relative to it's ground.
So is it possible to successfully read the values of the independently powered RGB LED with an Arduino?
Yes. There are multiple analog inputs.* The Uno has 6 analog inputs.
If so would one try reading the RGB LED values as a PWM or analog signal?
With a filter you can read the analog PWM average. That's a lot easier than "decoding" the PWM.
If there is a choice which would be best for lowest latency?
If you don't have any delays in your code it will respond in microseconds. (The filter will add some delay.)
How would one go about connecting the Arduino to the RGB LED?
A "regular little" RGB LED is connected just like 3 regular LEDs with 3 current-limiting resistors, Except, you do have to connect it correctly depending on if it's common-anode or common-cathode. (It's 3 LEDs with 4 wires coming out.)
High-power LEDs need a driver circuit. For an RGB, that's 3 constant-current** drivers.
An LED strip needs 3 MOSFET (or transistor) drivers (and usually a 12V power supply).
- There is one multiplexed (shared) analog-to-digital converter and there is some "settling time". When you change inputs, it's common to take an extra reading and throw-it away. The 2nd reading is more accurate. (This only takes microseconds.)
** Technically, controlled current if there is PWM dimming.