Mine

        gint color1_r, color1_g, color1_b, color2_r, color2_g, color2_b, min_d, d, color_index;

        GimpRGB bgcolor; if (!gimp_context_get_background(&my_color) { return 0; }

        color1_r = bgcolor.r
        color1_g = bgcolor.g
        color1_b = bgcolor.b

        min_d = 195075; /* Max value possible is R^2 + G^2 + B^2 = ( 255^2 ) * 3  */

        for (x = 0; x<16; x++) {

                color2_r = vicpalette[x*3]
                color2_g = vicpalette[x*3+1]
                color2_b = vicpalette[x*3+2]

                d = pow( (color1_r - color2_r), 2) ) + pow( (color1_g - color2_g) , 2 ) + pow ( (color1_b - color2_b), 2)
                if (d < min_d) (
                        min_d = d
                        color_index = x
                }
        }

        return min_d;

}