The heat has some effect, but it's only plays a small role..
It's definately oxidization...
I've experianced the same sort of thing.. early batches are yellow with a tint of green... later batches rust brown in colour.
Now I keep all my shit in air tight containers and in the freezer, and the color from batch to batch is the same more or less.. tren enan being more brownish in colour then tren ace. And that colour is determined primarily with how well and how long your product sat around at your suppliers.
as for black floaties, can't say I experienced that, but i have noticed a few batches of tren which did have more floaties and undissolvable contents then I would like to see, often making filtering much more difficult then it should be.