Is plutonium green

No, plutonium is a silvery grey metal that in subcritical quantities can glow with black body radiation, due to nuclear reactions heating the metal. This can glow anywhere from red, orange, yellow, white and blue (The amount of energy required to make something glow blue is actually quite a bit so it's rarely seen, except in stars). Black body radiation is never green. This is the same effect that happens when you heat any peice of metal, and it glows red hot. One special thing about plutonium is that when it is reacting within another medium, the radiation emitted by the plutonium can cause Cherenkov radiation, which is blue. It's caused by a particle traveling through a medium faster than the observed speed of light in that medium (The observed speed of light varies based upon the Index of Refraction for the material). A simple analogy is that Cherenkov radiation is a sonic boom for light. It also looks really cool.