You should be fine with painting your airsoft gun.
The short and quick of airsoft in Canada is, you can have one, you just cant import one. Although I believe in Ontario there is actually a law that states 18 to purchase, anyways that is the short and sweet, in reality its way more complicated.
In the USA in some states airsoft guns require an orange tip to identify them. That is not the case here in Canada. Treat your airsoft gun as if it were a real fire arm, proper storage and transportation techniques and you should be fine.
"Its only a little bit on fire"