Slider redS,greenS,blueS;
int red = 120;
int green = 120;
int blue = 120; 

void setup()
{
  size(500,500);
  background(red-120,green-120,blue-120);
  redS = new Slider(red,205);
  greenS = new Slider(green,235);
  blueS = new Slider(blue,265);
}
void draw()
{
 box();
 lines();
 sliders();
}

void box()
{
  rectMode(CORNER);
  noStroke();
  fill(190);
  rect(100,175,295,125); 
}

void lines()
{
  stroke(0);
  strokeWeight(3);
  line(120,205,335,205);
  line(120,235,335,235);
  line(120,265,335,265);
}

void sliders()
{
  redS.draw();
  greenS.draw();
  blueS.draw();
}
  
void mousePressed()
{
  while(redS.mouseOn()){
   redS.colour = mouseX;
  } 
}

class Slider{
  float colour,y;
  int w = 10;
  
  Slider(int colour, int y){
    this.colour = colour;
    this.y = y;
  }
  
  void draw()
  {
    fill(0,255,255);
    rectMode(CENTER);
    strokeWeight(1);
    rect(colour,y,w,w);
  }

  boolean mouseOn(){
   if(mouseX > colour && mouseX < colour+w && mouseY > y-w/2 && mouseY < y+w)
     {
      return true;
     }
    else {
     return false;
    }
  }
}