I use the blue mask if I'm planning to go above 18k and have been above 27k with no issues. (It is an unhelpful fire drill at 17k changing systems in a cockpit, so if you plan to go high, just start with the mask. You can fit it on the ground and hang it from the neck strap. Then pull the top strap back over the head when you get above 10k if you wish.)

It's not clear to me the regs or the physiology eliminate the cannula above 18k if you can remember to use your nose, but the mask covers the bases and looks cool in those wave picts.

What works can vary widely depending on the person, so pulse oximeter is useful to verify things are working. (A brain in la-la land is not dependable to do this on it's own. Especially in and above the 20's.)

Dress warm now...