Canadian Law dictates that it must be a clear body. A solid colour body can be interpretted as a real fire-arm from a distance. It is considered a prohibited item, and you're not licensed to import a prohibited item (And no, you can't just get licensed to import them. I've tried a million ways to get licensed, doesn't work).
It's also due to the fact that there's a serial number. You can't import any replica fire-arm goods with a serial number on it. At first glance by the CBSA, they see a metal reciever with a foreign serial number on it, they can assume that you're going to switch the serial number on your gun and never be tracked again. They could also potentially do some background checking to find out why you want this, since you don't own a firearms license and don't own an M4 (Or the AR-15 civilian model). Now you're busted for airsoft.
At least, that's what I'm led to understand.
The only way you can get a metal body is either purchase one that is already legally brought into the country, or find someone who does custom orders for prohibited items.
Primary: Custom WE M16A3
Secondary: KWA MP9
Side: Custom WE M1911A1