Isn't the yellow HEET methyl alcohol?
Isn't that poisonous? I'm worried about breathing in the vapors.

I'm using iso-heet now (red bottle) and wondering if I should switch over to the yellow bottle stuff.

Red bottle heet is isopropanol
Yellow bottle heet is methanol

Here is the msds for both:

Yellow Bottle

Red Bottle:

Neither of them do you want to be inhaling nor ingesting. As long as you are using both of them in a well ventilated area and do not pour it into your food or stick your face next to your stove you should be fine. Heck this is true for just about all the fuels. The only one I can think of that would be "ok" is pure ethanol but both ingesting and inhaling will get you drunk might quick.