What would the temperature be inside of a C02 canister?

As long as the CO2 canister is not being emptied or filled, it will be at the same temperature as the room that it is in. If you actively emptying the canister, the temperature can go drop significantly because of the drop in pressure (see Ideal Gas Law), and it can get hot if you are filling it. In general, if an object is in thermal equilibrium with it's surroundings (true as long you are not changing its temperature or the temperature of the surroundings at a significant rate), the object will be the exact same temperature as the surroundings.