Once you've found out, the difference between R, G & B, you can change the following code:
/*
In order to use the values from the sensor for the LED,
you need to do some math. The ADC provides a 10-bit number,
but analogWrite() uses 8 bits. You'll want to divide your
sensor readings by 4 to keep them in range of the output.
*/
redValue = redSensorValue/4;
greenValue = greenSensorValue/4;
blueValue = blueSensorValue/4;
The divider (4) should be replaced by a new one. Let's say, you have a perfect white with 50/255/50 on your RGB LED, meaning your Red and Blue should be 5 times weaker, you would change your code in this way:
redValue = redSensorValue/20;
greenValue = greenSensorValue/4;
blueValue = blueSensorValue/20;
The overall output of your RGB LED is now weaker.