Thanks Mike,
Much more responsive! Check this out:
var x, y, s, a, b, c, d, kol, s
function setup() {
	createCanvas(320, 200);
	background('black');
	s = 1000;
}
function mouseClicked() {
	if (s == 1000) {
		s = 5;
	}
	else {
		s += 5;
	}
	for (x = 0; x <= 320; x++) {
		for (y = 0; y <= 200; y++) {
			a = x * s / 600;
			b = y * s / 600;
			c = a * a + b * b;
			d = c / 2;
			d = d - Math.floor(d);
			d = d * 1000;
			if (d < 250) {
				kol = color(d, 0, 0);
			}
			else if (d < 500) {
				kol = color(0, d - 250, 0);
			}
			else if (d < 750) {
				kol = color(0, 0, d - 500);
			}
			else {
				kol = color(255, 255, 255);
			}
			stroke(kol);
			point(x, y);
		}
	}  
}
EDIT: missed a ; after d = d * 1000