Is this just another way of saying 'gravity is perpendicular to the ground'?

I don't think that's true.

Wikipedia:

> For example, Halon 1301 total flooding systems are typically used at concentrations no higher than 7% by volume in air, and can suppress many fires at 2.9% v/v. [...] Halon 1301 causes only slight giddiness at its effective concentration of 5%, and even at 15% those exposed remain conscious but impaired and suffer no long-term effects.

Also, Halon extinguishers don't work by displacing oxygen. They disrupt the chemical reaction of the fire itself by providing an alternative reaction.

Yeah, and then you get a really murky green and purple.

To me that mixture does look almost grey. But changing the exact choice of yellow and blue will change exactly what mixture they make. In any case it's much closer to grey than a 'primary' green would be.

It depends what colour mixing model you use. With paint it's true that yellow and blue mix to make green, but with light they instead make white (and your problem would instead be red and green mixing to make yellow). I was thinking of the Natural Color System in which red and green are opposed and yellow and blue are opposed.

I suppose the other solution would be to use lightness as well as hue. For example each mixture of the four emotions would correspond to a mixture of red, green, blue and white.

The colour yellow is associated with cowardice. So you could use that for fear, leaving green for joy.