def color_gradient(x):
    """
        Returns a color in Hex-RGB format between white and red if x is positive
        or between white and green if x is negative
    """
    grad = 220 - abs(x) * 80
    if x > 0:
        return '%.2x%.2x%.2x' % (255, grad, grad)
    elif x < 0:
        return '%.2x%.2x%.2x' % (grad, 255, grad)
    else:
        return '%.2x%.2x%.2x' % (100, 100, 100)
