Well if you mean melts from like sitting in the sun, dark chocolate, because the darker something is, the more pigments it has to absorb more color from the spectrum, thus absorbing more light, heating it up more, causing it to melt faster. Like how a black car gets hotter during the day than a white one, or when you wear a black shirt on a sunny day. As far as trying to melt it in like a pot or something, I'd have to say they all melt at the same rate. However, depending on how the chocolate is made, some chocolates will melt faster than others due to the ingredients. Like I would imagine harder chocolates would melt slower. Something you may want to look into is bakers chocolate, try to compare it to regular chocolate.