I am trying to use a LM324 4 channel opamp to create RGB color fading
This is a positive feedback Relaxation oscillator that drives one array only. i.e. it fades brightness in linear mode (although our eyes are logarithmic) and Red turns on first and off last but does not fade between colours.

It creates a variable triangle wave frequency biased from 9V to to give 0-30mA peak linear triangle current with series R to adjust colour brightness since green tends to be brighter but RED is 2.1 while B/G are 3.1V

You can adjust the pot or any part of the design using the mouse. (Ctrl+click to stretch parts, or drag around or edit values.... and more in this Falstad Java simulator



minor change to slow down effect with 1M pot. or increase C.