Parallel LED strings of different colors and Arduino
Looks fine to me. Be sure that the base resistor for each transistor is low enough that the transistor saturates. 1k is a good starting point. –  Dwayne Reid


My NFET suggestion reduces base/gate current from the MCU to "0".


  It looks good. There are ways to get more precise voltage. Really, LEDs are current driven devices, so current drivers are used as well. However, for simple projects, this is just fine. –  Justin Trzeciak


The point of all the fancy circuits/ICs is to maintain an equal "amount of light" with various colors, forward voltages, fluctuating inputs, high surge currents (output PWM), etc. It is obvious when one strand causes all of the others to fade out a little. For "simple projects", we don't care if they do.


  Consider using NFETs instead of Darlingtons. The strand resistors will need to vary from "ideal" to achieve the same "brightness" from each color; i.e. 10mA of red != 10mA of blue.


Red may need much more current than blue to be "equally bright".