如何用简单的算法生成一个类似『光盘』的彩色圆形图片？

# -*- coding:utf-8 -*-
from pylab import *

center = (250, 250)
radius = 250

img = zeros((500,500))

for i in range(500):
    for j in range(500):
        x = i - center[0] * 1.0
        y = j - center[1] * 1.0
        if x**2+y**2  radius**2:
            if x > 0:
                img[i,j] = arctan(y/x)
            elif x0 and y>=0:
                img[i,j] = arctan(y/x) + pi
            elif x0 and y0:
                img[i,j] = arctan(y/x) - pi
            elif x==0 and y>0:
                img[i,j] = pi / 2
            elif x==0 and y0:
                img[i,j] = pi / -2
            elif x==0 and y==0:
                img[i,j] = 0.0

print img
imshow(img, cmap=cm.hsv)
show()