Hue.

Hue is the color reflected from or transmitted through an object and is typically referred to as the name of the color (red, blue, yellow, etc.) Move the cursor vertically over each bar to alter its hue.

 
int barWidth = 5;
int[] hue;

void setup() 
{
  size(400, 400);
  colorMode(HSB, 360, height, height);  
  hue = new int[width/barWidth];
  noStroke();
}

void draw() 
{
  int j = 0;
  for (int i=0; i<=(width-barWidth); i+=barWidth) {  
    if ((mouseX > i) && (mouseX < i+barWidth)) {
      hue[j] = mouseY;
    }
    fill(hue[j], height/1.2, height/1.2);
    rect(i, 0, barWidth, height);  
    j++;
  }
}