The color is the result of phosphorescence. The best compounds available emitted green color only.
The "light" in CRTs is due to the impact of electron beams on a chemical (the phosphor) that is quickly excited to a higher energy state and then releases that energy in the form of light as it 'relaxes' to its original energy state.
The release of energy is a quantum amount - only specific wavelengths (energy) are emitted for the specific energy difference between the atomic ground and excited states of the phosphor.
It took longer (and was more expensive) to develop techniques and materials to produce different-colored emissions. For many applications of CRTs in science and technology a monochrome screen is sufficient to this day.