Someone correct me if I'm wrong, but B50 stretches much more than B500. Mix the two together and you're going to have a lot more pressure on the B500 strands than on the B50, eventually resulting in a snapped string. I use black B450 (essentially the same thing as B500 I think) and add an extra two coloured strands of B50 just for looks. I wouldn't make a string consisting of equal parts B50-B500, or give the B50 and weight bearing load when mixed with B500. Most of the weight will go to the B500 and that won't end up too well.