Adjusting the Color of an RGB LED. https://techzeero.com/arduino-tutorials/adjusting-the-color-of-an-rgb-led/

Adjusting the Color of an RGB LED.ino

/* 
   Adjusting the Color of an RGB LED
   For more details, visit: https://techzeero.com/arduino-tutorials/adjusting-the-color-of-an-rgb-led/
*/

const int redPin = 3; // choose the pin for each of the LEDs
const int greenPin = 5;
const int bluePin = 6;
const boolean invert = false; // set true if common anode, false if common cathode
int color = 0; // a value from 0 to 255 representing the hue
int R, G, B; // the Red Green and Blue color components

void setup()
{
  // pins driven by analogWrite do not need to be declared as outputs
}

void loop()
{
  int brightness = 255; // 255 is maximum brightness
  hueToRGB( color, brightness); // call function to convert hue to RGB
  // write the RGB values to the pins
  analogWrite(redPin, R);
  analogWrite(greenPin, G);
  analogWrite(bluePin, B );
  color++; // increment the color
  if(color > 255)
    {
      color = 0;
    }
  	delay(20);
}

// function to convert a color to its Red, Green, and Blue components.
void hueToRGB( int hue, int brightness)
{
  unsigned int scaledHue = (hue * 6);
  unsigned int segment = scaledHue / 256; // segment 0 to 5 around the color wheel
  unsigned int segmentOffset = scaledHue - (segment * 256); // position within the segment
  unsigned int complement = 0;
  unsigned int prev = (brightness * ( 255 - segmentOffset)) / 256;
  unsigned int next = (brightness * segmentOffset) / 256;

  if(invert)
  {
    brightness = 255-brightness;
    complement = 255;
    prev = 255-prev;
    next = 255-next;
  }
  switch(segment) 
    {
    case 0: // red
      R = brightness;
      G = next;
      B = complement;
    break;
    
      	case 1: // yellow
      R = prev;
      G = brightness;
      B = complement;
    break;

      	case 2: // green
      R = complement;
      G = brightness;
      B = next;
    break;

      	case 3: // cyan
      R = complement;
      G = prev;
      B = brightness;
    break;

      	case 4: // blue
      R = next;
      G = complement;
      B = brightness;
    break;
    case 5: // magenta
    default:
      R = brightness;
      G = complement;
      B = prev;
    break;
  }
}