There are also rather a lot of chemical reactions taking place in a body while it's alive. That's what life is. Loss of oxygen supply is not particularly likely to cause greater release of energy, is it? The energy releasing processes in the body require oxygen.
Body loses heat on death, I can't say whether overall effect is exothermic or endothermic. I also can't say whenther overall effects of all changes on death, is release of photons or absorption of photons due to chemical changes in body on death. Can you tell this? Since you anticipate only heat or black body radiations but not light , I don't think just heat loss can cause any soul/ghost effect because nothing is specific & chracteristic in this. Moreover, souls are considered bit as cool entity.
The colour change you mention is caused by decomposition (also possibly by the blood settling - I thnk this may be referred to as lividity?) which is caused by the life processes of bacteria etc. - not really anything to do with the person who's just died.
Instant colour changes on death, I think can be just by loss of blood colour & supply. This change in outer colour of body can effect emission/reflection pattern--if body is exposed to light at that time.
What has this got to do with "souls" or "ghosts?
It is indicated that Re-birth can be in several living beings(somewhat 80000 type of living beings). Furthur, in spritual sayings, various aspects might had been indicated differently as by examples/stories or in differant languages, which in actuals can be in different real form. We need to understand those in reality or in concept not in person.
"While we are alive, we do not emit or reflect light any differently to any other object of a similar temperature and colour. Do you remember what you've been told about black body radiation? (apparently not)
We anticipate only heat/black body radiation as body energy, but there can also be some energies as energy in visible spectrum due to change in colours of our exposed parts.
What has this got to do with "souls" or "ghosts?"
One thought can be that soul/ghost is an imaginary entity shown/indicated of what we have left behind. Our children & our effects-good or bad deeds etc. are our remained effects, which we are leaving behind us. So what this world will gain or loose from these remains--may be somewhat thought as our soul/ghost as a concept not a true presense of any physical or energetic entity.