I think the word "yellow" stands for the part about love that we cannot explain. Yellow stands for the part where it cannot be described or contained, even within itself. When he says, "your skin and bones turn into something beautiful", hes talking about the fact that humans are just skin and bones, and yet beauty inexplicably comes out of it. Just like the sun: its just a big piece of burning gas, but damn it, it is yellow and beautiful :)