There is no fancy code required, the ZeRGBa widget can output three values directly to PWM capable pins on an Arduino or similar device with supported PWM pins - one each for RED, GREEN & BLUE. And the custom 1602 LCD w/ RGB backlight has three backlight input pins... you guessed it, one each for RED, GREEN & BLUE (and Common Anode - power).
You can also opt for virtual pins and add some code if you want more flexibility.
Simply search for the word zeRGBa in this forum for many examples of code to control RGB LED's and even virtual LEDS and even the "backlight" colours on the LCD and other display widget colours on the App.
This one was easy to provide as I had already posted it many times, but please remember, it is generally prefered if people do some searching and research on their own before asking for code.