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Arduino sketch to cycle an RGB LED through the colour spectrum.
const int redPin = 11;
const int greenPin = 10;
const int bluePin = 9;
void setup() {
// Start off with the LED off.
void loop() {
unsigned int rgbColour[3];
// Start off with red.
rgbColour[0] = 255;
rgbColour[1] = 0;
rgbColour[2] = 0;
// Choose the colours to increment and decrement.
for (int decColour = 0; decColour < 3; decColour += 1) {
int incColour = decColour == 2 ? 0 : decColour + 1;
// cross-fade the two colours.
for(int i = 0; i < 255; i += 1) {
rgbColour[decColour] -= 1;
rgbColour[incColour] += 1;
setColourRgb(rgbColour[0], rgbColour[1], rgbColour[2]);
void setColourRgb(unsigned int red, unsigned int green, unsigned int blue) {
analogWrite(redPin, red);
analogWrite(greenPin, green);
analogWrite(bluePin, blue);

Wouldn't it be better to use PWM here? Leds have a nonlinear voltage response

Well written jamesotron, thanks.

Peterrus: analogWrite() is PWM.

razic commented Sep 12, 2013

Perfect. Just what I was looking for!

stonez56 commented Jan 7, 2014

Very well written codes! Thank you so much!!

Neat code! Thank you!

Yeah, thanks!)

Hello Arduino World,
I'm trying to work out how this sketch works. I understand it up to the line 20.
int incColour = decColour == 2 ? 0 : decColour + 1;
I think it starts by declaring an integer called incColour and sets it equal to the other int decClour but then I get lost with the two = signs and then the quesion mark. Is there a long way to write this line that will be easier to understand?

pizen commented Oct 25, 2015


? is a ternary operator. Line 20 is the same logic as:

int incColour;
if (decColour == 2) {
    incColour = 0;
} else {
    incColour = decColour +1;


You can skip a block of code by assigning values when you initialize the array...

unsigned int rgbColor[3] = {255, 0, 0};

Also, thanks for this example.

You can also write that line as:
incColour = (decColour + 1) % 2

Cellane commented Oct 10, 2016

@ManciuStefan Not really, that line only ever gives you 0 or 1 as an output. The original line with ternary operator gives you either 0, 1, or 2.

nizamky commented Oct 11, 2016

i need to control rgb light via blutooth from mobile?. any advice?

@jamesotron Your schetche works but, you need to explain stuff and add comments. Explain what this means(on line 20):

int incColour = decColour == 2 ? 0 : decColour + 1;

This works great for common cathode LEDS.
If you're using common Anode LEDS, change line 27 to:
setColourRgb(255-rgbColour[0], 255-rgbColour[1], 255-rgbColour[2]); //quick and ugly invert

Interesting code, but for me only blue changes

jdimpson commented Jan 9, 2018

I just noticed a minor bug. A stickler (such as myself) may wish to call setColourRgb(rgbColour[0],rgbColour[1],rgbColour[2]); delay(5); immediately after rgbColour gets initialized to {255,0,0}, before hitting the for loops. Otherwise the LED doesn't ever hit a pure red output.

Alternatively, you could initialize rgbColour to {256,-1,-1}, but then you'd need to change the declaration of rgbColour to signed and make similar change to the arguments to setRgbColour() function. I'm pretty sure making it signed won't hurt anything; at worst analogRead() will complain about implicit casting. If that bothers you (and it should) you can safely cast it to unsigned in setRgbColour()

I noticed that the red output seemed underpowered a few months ago, but assumed I used too large a value of resistor on the red pin. That may still be true, but this fix should help a bit.

I love this gist, by the way. I've come back to it several times since I first saw it a couple years ago. Thank you, @jamesotron !

michalmonday commented Feb 8, 2018

Simultaneous increasing and decreasing of 2 colours leads to the situation where the colors "meet" at midpoint between 0 and 255 so the RGB status is like 127,128,0 which gives a darker shade. If you take a look at "color picker" in google, set the color to be the brightest and play with the slider you'll notice that it always changes 1 of the RGB variables at a time leading it to reach 6 main states that gradually switch between themselves, these states are:

  1. 255, 0, 0
  2. 255, 255, 0
  3. 0, 255, 0
  4. 0, 255, 255
  5. 0, 0, 255
  6. 255, 0, 255
  7. 255, 0, 0 (whole cycle ends, it's the same as 1st step)

Your implementation is providing the following states:

  1. 255, 0, 0
  2. 127, 128, 0
  3. 0, 255, 0
  4. 0, 128, 127
  5. 0, 0, 255
  6. 127, 0, 128
  7. 255, 0, 0

Here's a python implementation which shows how to loop through RGB without getting the darker shade:

I guess it would be better without the section list but it does the thing... On the other side it allows for other operations being executed without the need to complete the whole cycle from red to blue, also it would be easy to implement custom sequences of colours with it.

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