Algorithm for altering the saturation of a single color. (not using HSB mode).

saturation.pde

color col;

void setup() {
  size(200, 100);
  col = color(50, 100, 25);
}

void draw() {
  background(0);
  noStroke();
  fill(col);
  rect(0, 0, 100, 100);

  float sat = map(mouseX,0,width,0,500);

  float r = red(col);
  float g = green(col);
  float b = blue(col);

  float r1 = r; 
  float g1 = g; 
  float b1 = b;
  float ry1 = (( 70*r1-59*g1-11*b1)/100);
  float by1 = ((-30*r1-59*g1+89*b1)/100);
  float gy1 = ((-30*r1+41*g1-11*b1)/100);
  float y   =  ( 30*r1+59*g1+11*b1)/100 ;
  float ry  = (ry1*sat)/100;
  float gy  = (gy1*sat)/100;
  float by  = (by1*sat)/100;
  r1  = ry + y; 
  g1 = gy + y; 
  b1 = by + y;
  r1 = constrain(r1, 0, 255); 
  g1 = constrain(g1, 0, 255); 
  b1 = constrain(b1, 0, 255);
  r = r1; 
  g = g1; 
  b = b1;
  
  fill(r,g,b);
  rect(100,0,100,100);

}