Which chocolate melts fastest dark, white or milk. And Why?

Dark things absorb more heat from sunlight than light things

I believe there are a few factors to consider. The main one dealing with heat is introduced and how it moves into things to cause it to melt. Heat will melt things via conduction and convection, and the major force at work will be conduction. The conduction factor of heat moving into your chocolate will vary, and you suspect that the composition factors in, which it does as far as the ingredients. What you really want to research is what kind of structure the chocolate is and how it affects conduction. The same factors that make heat pass through metal faster than glass is at work. Chocolate is tempered. White chocolate will likely melt slower because it is more amorphous and less crystalline, as well as having a higher specific heat to absorb. Milk chocolate is tempered and specifically made by design to crystallize within a certain temperature range, and to melt within a certain temperature range. Dark chocolate has different ingredients and is tempered to crystallize differently as well. There is either an ingredient added or missing that is impacting the crystal structure, and how fast it transfers heat and affects how fast it melts. You'll also want to compare the ingredients to see if any of the ingredients can improve the heat resistance of chocolate such as a substitute for cocoa butter. I've linked some useful info.

Well I would think it depends on the brand of chocolate you use. ...A brand higher in fat and oil would melt quicker. I think dark chocolate melts faster because it attracts more heat.

actually white chocolate melts faster. I know this because I have worked with both dark and white chocolates.

Generally white, because it has a higher fat content. In fact it is not truly chocolate.