Brightness

int barWidth = 20;
int lastBar = -1;

void setup() {
    size(640, 360);
    colorMode(HSB, width, 100, height);
    noStroke();
    background(0);
}

void draw() {
    int whichBar = mouseX / barWidth;

    if (whichBar != lastBar) {
        int barX = whichBar * barWidth;

        fill(barX, 100, mouseY);
        rect(barX, 0, barWidth, height);

        lastBar = whichBar;
    }
}