import math


def COL_DIST2(col1, col2):
    """
    Computes 2-distance between two RGB vectors, i.e.
        = sqrt(r^2 + g^2 + b^2)
    """
    r, g, b = (col1[i] - col2[i] for i in range(0, 3))
    return math.sqrt(r * r + g * g + b * b)