The painted end is not a reference as different mfg's can use paint to mean different things. Assuming that after the tube is mitered there is a "long" and "short butt, the long one usually goes to the head tube. Most round tubes have butts of equal wall thickness at both ends- when you say "thicker" end, do you mean longer butt, or an actually thicker wall? If you mean thicker wall make sure you haven't just turned a seat tube into a own tube.