'''
based on: http://24ways.org/2010/calculating-color-contrast/
accepts hex colors with 3 or 6 chars. hash-char is optional.
'''
def get_contrast_yiq(hex_color):
    hex_color = hex_color.replace("#", "")

    if len(hex_color) == 3:
        hex_color = "".join([c + c for c in hex_color])
    
    r = int(hex_color[0:2], 16)                                                                                                                                                           
    g = int(hex_color[2:4], 16)                                                                                                                                                           
    b = int(hex_color[4:6], 16)
    
    yiq = ((r * 299) + (g * 587) + (b * 114)) / 1000

    if yiq >= 128:
        return 'black'
    return 'white'