How to Use an RGB LED: Color Mixing with Arduino, Common Anode vs Cathode

An RGB LED is three separate LEDs in a single package: red, green and blue. By setting the brightness of each one independently, you can make almost any color you like, from yellow to cyan and from purple to orange. Every pixel on a computer screen works on the same principle. It's one of the most fun Arduino parts for status lights, mood lamps and colourful notification indicators.

How does an RGB LED work?

Colours of light mix additively: red and green together look yellow, green and blue give cyan, red and blue give purple (magenta), and all three together make white. The Arduino sets each channel's brightness from 0 to 255 using PWM. That gives, in theory, 256 × 256 × 256 ≈ 16.7 million combinations. PWM switches the LED on and off hundreds of times per second; your eye can't see the flicker and only perceives the average brightness.

Common cathode or common anode?

The longest of the RGB LED's four legs is the common pin shared by the three LEDs. If it joins the LEDs' cathodes, the LED is common cathode; if it joins their anodes, it's common anode. They look identical from the outside, but their wiring and code are opposite:

Pin order and Arduino wiring

On common 5 mm RGB LEDs the pin order is red, common (longest), green, blue, but because it can vary between manufacturers, testing each leg is the safest bet. Each color connects through its own resistor to a PWM pin. On the Arduino Uno the PWM pins are the ones marked with ~: 3, 5, 6, 9, 10 and 11.

Resistor values: a separate resistor for each color

Each color needs a different voltage to light up (its forward voltage): about 2.0 V for red and 3.0–3.2 V for green and blue. Aiming for 20 mA at 5 V, red needs (5 − 2.0) / 0.020 = 150 Ω and green and blue need (5 − 3.2) / 0.020 ≈ 100 Ω. In practice, fitting 220 Ω on all three is safe and still bright enough; you can balance the colors in code. To calculate your own values, use the LED resistor calculator, and for each color's forward voltage, the LED voltages table.

Mixing colors with Arduino

The code below wraps the three channels in a single setColor() function and shows seven colors in turn. If your LED is common anode, just set COMMON_ANODE to true; the function inverts the values for you.

1const int R_PIN = 9;
2const int G_PIN = 10;
3const int B_PIN = 11;
4const bool COMMON_ANODE = false;   // ortak anotsa true yap / set true for common anode
5
6void setColor(int r, int g, int b) {
7  if (COMMON_ANODE) { r = 255 - r; g = 255 - g; b = 255 - b; }
8  analogWrite(R_PIN, r);
9  analogWrite(G_PIN, g);
10  analogWrite(B_PIN, b);
11}
12
13void setup() {
14  pinMode(R_PIN, OUTPUT);
15  pinMode(G_PIN, OUTPUT);
16  pinMode(B_PIN, OUTPUT);
17}
18
19void loop() {
20  setColor(255, 0, 0);     delay(1000);  // kırmızı / red
21  setColor(0, 255, 0);     delay(1000);  // yeşil / green
22  setColor(0, 0, 255);     delay(1000);  // mavi / blue
23  setColor(255, 255, 0);   delay(1000);  // sarı / yellow
24  setColor(0, 255, 255);   delay(1000);  // turkuaz / cyan
25  setColor(255, 0, 255);   delay(1000);  // mor / magenta
26  setColor(255, 255, 255); delay(1000);  // beyaz / white
27}

Smooth color transitions: a rainbow effect

Instead of jumping between colors, if you decrease one channel while increasing another, the light flows smoothly from red to green, green to blue and back to red. The loop below uses the same setColor() function; change the delay value to adjust the speed.

1// setColor() ve pin tanımları yukarıdaki gibi / setColor() and pins as above
2void loop() {
3  for (int i = 0; i < 256; i++) { setColor(255 - i, i, 0); delay(8); }  // kırmızı → yeşil / red → green
4  for (int i = 0; i < 256; i++) { setColor(0, 255 - i, i); delay(8); }  // yeşil → mavi / green → blue
5  for (int i = 0; i < 256; i++) { setColor(i, 0, 255 - i); delay(8); }  // mavi → kırmızı / blue → red
6}

Choosing a color with potentiometers

If you connect three potentiometers to A0, A1 and A2, you can use each one as the knob for one color channel. analogRead() returns a value from 0 to 1023; dividing it by 4 gives the 0–255 range PWM expects. Wiring a potentiometer is explained in What Is a Potentiometer?

1// A0, A1, A2'de üç potansiyometre; setColor() yukarıdaki gibi
2// three pots on A0, A1, A2; setColor() as above
3void loop() {
4  int r = analogRead(A0) / 4;   // 0–1023 → 0–255
5  int g = analogRead(A1) / 4;
6  int b = analogRead(A2) / 4;
7  setColor(r, g, b);
8}

RGB LEDs with the ESP32 and Raspberry Pi Pico

On the ESP32 with the current Arduino core (3.x), analogWrite() works on any output pin, so the code above runs as-is once you change the pin numbers. The one thing to watch is voltage: the pins output 3.3 V, so the green and blue channels, whose forward voltage is close to 3 V, are dimmer. At 3.3 V use small resistors such as 47–100 Ω, or drive the LED from 5 V through a transistor if you want it really bright. On the Pico, MicroPython's PWM class takes values from 0 to 65535:

1from machine import Pin, PWM
2import time
3
4pins = [PWM(Pin(n)) for n in (11, 12, 13)]   # GP11 R, GP12 G, GP13 B
5for p in pins:
6    p.freq(1000)
7
8def set_color(r, g, b):                       # 0–255
9    for p, v in zip(pins, (r, g, b)):
10        p.duty_u16(v * 257)                   # 0–65535 (ortak anotta 65535 - v * 257)
11
12while True:
13    set_color(255, 0, 0); time.sleep(1)       # kırmızı / red
14    set_color(0, 255, 0); time.sleep(1)       # yeşil / green
15    set_color(0, 0, 255); time.sleep(1)       # mavi / blue

Regular RGB LED or addressable LED (WS2812B)?

For one or two colored lights, a regular RGB LED is the simplest and cheapest solution. If you want to color dozens of LEDs individually, the Uno's 6 PWM pins aren't enough; that's when you move to WS2812B strips, which control hundreds of LEDs over a single wire.

RGB LED project ideas

• Mood lamp: put the rainbow fade inside a frosted jar for a soft night light.
• Temperature indicator: combine it with a DHT11 for a lamp that glows blue when it's cold, green when it's mild and red when it's hot.
• Color mixer: create any color by hand with three potentiometers.
• Pick a color from your phone: change the lamp's color from a color picker on a web page served by an ESP32.
• Notification light: flash the LED in a particular color when a new email or message arrives.
• Keypad colors: show a different color depending on the number typed on a keypad.

Common mistakes and how to fix them

• The colors work backwards: you're driving a common-anode LED as if it were common cathode; set COMMON_ANODE to true.
• One color never lights: the pin order may differ from what you expected; test each leg one by one with a resistor.
• Fades are stepped, with no in-between shades: the LED is on a non-PWM pin; on the Uno those pins can only be fully on or off.
• Colors glitch while a buzzer plays: on the Uno, tone() uses Timer2 and breaks PWM on pins 3 and 11; move the RGB LED to pins such as 5, 6, 9 and 10.
• Leaving out the resistors: too much current eventually damages both the LED and the Arduino pin.
• Very dim on the ESP32: at 3.3 V green and blue are at their limit; use smaller resistors or a transistor.

If you want to understand LED basics and why they need resistors, take a look at What Is an LED? and What Is a Resistor? In the BlueGrays app you'll find RGB LED color-fade, keypad-controlled lamp and color-mixer projects with circuit diagrams and code.