This example is for Processing 3+. If you have a previous version, use the examples included with your software. If you see any errors or have suggestions, please let us know.

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 = 20;
int lastBar = -1;

void setup() 
{
  size(640, 360);
  colorMode(HSB, height, height, height);  
  noStroke();
  background(0);
}

void draw() 
{
  int whichBar = mouseX / barWidth;
  if (whichBar != lastBar) {
    int barX = whichBar * barWidth;
    fill(mouseY, height, height);
    rect(barX, 0, barWidth, height);
    lastBar = whichBar;
  }
}