rgb.c

int B = 3;
int G = 5;
int R = 6;

int red = 255;
int green = 0;
int blue = 0;

int step = 1;

int phase = 0;

void setColor(int r, int g, int b) {
  analogWrite(R,r);
  analogWrite(G,g);
  analogWrite(B,b);
}

void setup() {
  pinMode(B, OUTPUT);
  pinMode(G, OUTPUT);
  pinMode(R, OUTPUT);
}

void loop() {
  
  if(green == 255 && red == 0) {
    phase = 1;
  }
  
  if(blue == 255 && green == 0) {
    phase = 2;
  }
  
  if(red == 255 && blue == 0) {
    phase = 0;
  }
  
  if(phase == 0) {
    red -= step;
    green += step;
  }
  
  if(phase == 1) {
    green -= step;
    blue += step;
  }
  
  if(phase == 2) {
    blue -= step;
    red += step;
  }
  
  setColor(red, green, blue);
  delay(5);
}

rgb_button.c

int B = 3;
int G = 5;
int R = 6;
int button = 4;
int led = 13;

int red = 255;
int green = 0;
int blue = 0;

int step = 1;

int phase = 0;

void setColor(int r, int g, int b) {
  analogWrite(R,r);
  analogWrite(G,g);
  analogWrite(B,b);
}

void setup() {
  pinMode(B, OUTPUT);
  pinMode(G, OUTPUT);
  pinMode(R, OUTPUT);
  pinMode(led, OUTPUT);
  pinMode(button, INPUT);
}

void loop() {
  
  int stopBlink = digitalRead(button);
  digitalWrite(led, stopBlink);
  
  if(green == 255 && red == 0) {
    phase = 1;
  }
  
  if(blue == 255 && green == 0) {
    phase = 2;
  }
  
  if(red == 255 && blue == 0) {
    phase = 0;
  }
  
  if(phase == 0) {
    red -= step;
    green += step;
  }
    
  if(phase == 1) {
    green -= step;
    blue += step;
  }
    
  if(phase == 2) {
    blue -= step;
    red += step;
  }
    
  if(stopBlink == LOW) {
    setColor(0, green, blue);
  } else {
    setColor(red, green, blue);
  }
  
  delay(5);
}