Radioactive materials are often portrayed as green because back in the mid 20th century, there was a trend of making glowing watch faces with radioactive paint mixed with phosphorus. As you say, radioactivity is impossible to see with the naked eye and if you have Cherenkov radiation, it's a visible blue. The phosphorus interacting with the radioactive decay is what makes a green glow. As time went on, the glow of the phosphorus was associated with radiation. As was the case with these watch faces. Some were even made with radium.

These sorts of watches were pretty popular and had to be hand painted. Sadly the women who made them had horrible amounts of cancer from working with all these radioactive materials.

Source: http://www.abc.net.au/science/articles/2008/05/20/2249925.htm