RGB LED:

    // Define Pins
    #define BLUE 4
    #define GREEN 5
    #define RED 2
    
    // define variables
    int redValue;
    int greenValue;
    int blueValue;
    
    void setup()
    {
    pinMode(RED, OUTPUT);
    pinMode(GREEN, OUTPUT);
    pinMode(BLUE, OUTPUT);
    digitalWrite(RED, HIGH);
    digitalWrite(GREEN, LOW);
    digitalWrite(BLUE, LOW);
    }
    
    void loop()
    {
    // fading time between colors
    #define delayTime 5
    
    redValue = 255; // choose a value between 1 and 255 to change color's brightness.
    greenValue = 0;
    blueValue = 0;
    
    // fades out red; fade in green
    for(int i = 0; i < 255; i += 1)
    {
    redValue -= 1;
    greenValue += 1;
    analogWrite(RED, redValue);
    analogWrite(GREEN, greenValue);
    delay(delayTime);
    }
    
    redValue = 0;
    greenValue = 255;
    blueValue = 0;
    
    // fades out green; fade in blue
    for(int i = 0; i < 255; i += 1)
    {
    greenValue -= 1;
    blueValue += 1;
    analogWrite(GREEN, greenValue);
    analogWrite(BLUE, blueValue);
    delay(delayTime);
    }
    
    redValue = 0;
    greenValue = 0;
    blueValue = 255;
    
    // fades out blue; ..and again fade in red.
    for(int i = 0; i < 255; i += 1)
    {
    blueValue -= 1;
    redValue += 1;
    
    analogWrite(BLUE, blueValue);
    analogWrite(RED, redValue);
    delay(delayTime);
    }
    }