Python (using graphics.py):
from graphics import *
xmax = 800
ymax = 600
win = GraphWin('Circles', xmax, ymax)
win.autoflush = False
win.getMouse()
while 1:
	s = 500
	for x in range(0, xmax + 1):
		for y in range(0, ymax + 1):
			a = x * s / 600.0
			b = y * s / 600.0
			c = a * a + b * b
			d = c / 2.0
			d = d - int(d)
			if d < 0.25:
				kol = color_rgb(d * 4, 0, 0)
			elif d < 0.5:
				kol = color_rgb(0, 2 * d, 0)
			elif d < 0.75:
				kol = color_rgb(0, 0, 4 / 3.0 * d)
			else:
				kol = color_rgb(255, 0, 0)
			win.plot(x, y, kol)		
		print x
	win.flush()
	win.getMouse()
	if s == 1000:
		s = 500
	else:
		s += 5