It's starting to get more complicated as I research it. There are 3 sources of heat loss.

1. Conduction
2. Radiation
3. Convection

Convection is the biggest and it's mostly out around the shoulders. There may be some along the zipper.

Radiation is the heat that rises off your body. The best idea is to reflect it back with light colored material. When picking a color for long johns, the lighter the better. Then cover them with a dark shirt or fleece during the day so radiation works in your favor.

A light color for the top of the sleeping bag will radiate less heat than a dark color. The inside of my old Coleman is a pale yellow which may partially explain why it's warm.

Rather than try to figure out the science, tonight on the hammock, I'm going to put a ridgeline inside the sleeping bag to see what happens. My theory is my body will heat the air around me better than it would heat the sleeping bag which is higher density. If it makes me colder, then I'll save it for warmer weather.