Hydrogen emission is red. Oxygen is blue-green. These are true colors and make up the bulk of emission. Other elements do produce colors, neon is red too, for instance. Emission happens when gases are ionized and then recombine--excited electrons leave the atoms, then rejoin. Photons of characteristic color/energy are released upon recombination, hence the red of hydrogen and blue-green of oxygen. Hydrogen is common in "stellar nursery" nebulas, hence they're mostly reddish. Planetary nebulas often show a lot of green due to doubly ionized oxygen.

The so-called Hubble palette is actually false color, produced with narrowband filters--sulfur is shown as red, hydrogen as blue, oxygen as green.