rainbow.py

import sys 

text = sys.argv[1]

def hue(angle):
    h = angle / 60
    x = 1 - abs(h % 2 - 1)
    return ((1, x, 0), (x, 1, 0), (0, 1, x), (0, x, 1), (x, 0, 1), (1, 0, x))[int(h)]

def hexcolor(rgb):
    return '#{:02x}{:02x}{:02x}'.format( * (int(255 * v) for v in rgb) )

print(''.join('<span style=\'color: {0}\'>{1}</span>'.format((hexcolor(hue(i * 360 / len(text)))), character) for i, character in enumerate(text)))