A RGB LED is a LED that can change the colour of the light it produces depending on which of the LEDs’ Connectors have current flowing through them. The LED has 4 connectors, one connector for red, one for green, one for blue and then finally an anode or a cathode, depending if the RGB is a common anode or cathode LED.

So what is the difference between common anode and common cathode?

Well a RGB LED is actually a combination of 3 LEDs, a red LED, a green LED and a blue LED. All LEDs have 2 connectors, an anode and a cathode. So depending how these LEDs are connected together determines if they share an anode or a cathode, thus common anode RGB LED or common cathode RGB LED. The Anode\Cathode leg can be identified as it is the longest leg on the LED. Below are 2 diagrams that illustrates the difference discussed.

Common Cathode:

Common Anode:

How these 2 different RGB LEDs are connected to a circuit also differs, let us first have a look at a circuit that contains a common cathode RGB LED:

Here is the code used with this circuit:

int redPin = 9;
int greenPin = 10;
int bluePin = 11;
 
void setup()
{
  pinMode(redPin, OUTPUT);
  pinMode(greenPin, OUTPUT);
  pinMode(bluePin, OUTPUT);  
}
 
void loop()
{
  setLEDColour(255, 0, 0);  // red
  delay(2000);
  setLEDColour(0, 255, 0);  // green
  delay(2000);
  setLEDColour(0, 0, 255);  // blue
  delay(2000);
  setLEDColour(255, 255, 0);  // yellow
  delay(2000);  
  setLEDColour(80, 0, 80);  // purple
  delay(2000);
}
 
void setLEDColour(int red, int green, int blue)
{
  analogWrite(redPin, red);
  analogWrite(greenPin, green);
  analogWrite(bluePin, blue);  
}

Now let us have a look at a circuit that contains a common anode RGB LED:

Code used with this circuit:

int redPin = 11;
int greenPin = 10;
int bluePin = 9;
 
void setup()
{
  pinMode(redPin, OUTPUT);
  pinMode(greenPin, OUTPUT);
  pinMode(bluePin, OUTPUT);  
}
 
void loop()
{
  setLEDColour(255, 0, 0);  // red
  delay(2000);
  setLEDColour(0, 255, 0);  // green
  delay(2000);
  setLEDColour(0, 0, 255);  // blue
  delay(2000);
  setLEDColour(255, 255, 0);  // yellow
  delay(2000);  
  setLEDColour(80, 0, 80);  // purple
  delay(2000);
}
 
void setLEDColour(int red, int green, int blue)
{
  red = 255 - red;
  green = 255 - green;
  blue = 255 - blue;

  analogWrite(redPin, red);
  analogWrite(greenPin, green);
  analogWrite(bluePin, blue);  
}

Although the circuits and code differ between the 2 types of RGB LEDs, the end results are exactly the same.

Try changing the values passed into the setLEDColour function to see what different colours can be created.