First, we must understand that phosphorescence is what makes a glow-in-the-dark thingy glow. And the most common, most active phosphorescing chemical is zinc sulfide, which "glows" green. It has other benefits in that it is nontoxic and not radioactive. It can also be "charged" with ordinary, visible light bulb light. What, exactly, are the advantages of green, though?
The human eye is maximally sensitive to green. Thus a phosphor emitting a given optical power in the green will be perceived as being brighter than a phosphor of any other color with the same emitting power.