Hue

/**
 * Hue. 
 * 
 * Hue is the color reflected from or transmitted through an object 
 * and is typically referred to as the name of the color such as 
 * red, blue, or yellow. In this example, 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;

        // No casting, raw version
        fill(mouseY, height, height);

        rect(barX, 0, barWidth, height);
        lastBar = whichBar;
    }
}