This is my understanding of it.
It's not that no combination of gasses can make green, it's all about temperature. When something is hot enough to glow in the visible spectrum, it starts off red, then goes to white, then blue. The reason it's white is because it isn't one colot like a neon sgin or a laser, but a whole range of them that peaks at the color we call it. A "red" is mostly infra-red, then some red and some orange and yellow. We can't see the infra-red, so see wee the second most dominant color. The red. It washed out the yellow and orange.
For a start to be green, it would radiate almost the entire visible spectrum as its range. When you add all those colors together, you get white. This is compounded by the fact that human eyes or not all that sensitive to green light as compared to the other colors.
See below for corrections.
I'm Not Evil.
An evil person would do the things that pop into my head.