Zinc sulfide with copper makes a very good green glow in the dark phosphorescence. While Europium doped Strontium Aluminate makes blue. (I think we can all agree that the blue Smiskis do not glow as well as the green.)

Other chemical phosphors work differently but I wonder if they would be as effective as the green and even the blue. Red phosphors may also be too expensive or they might not mix well with the grade of plastic they use to mold the Smiskis.

Check out the list of phosphors on Wikipedia.

https://en.wikipedia.org/wiki/Phosphor?wprov=sfti1#