First, go into the settings and look for "Allow Developer Console" or whatever it's called.
Then, press the key underneath the Escape* key (the one that looks like a ~ with a ` underneath it) to open the console.

Once you have the paint gun, use the command to give yourself a portal gun-infused paint gun.

Once you have the portal gun-infused paint gun, use the command to get blue and orange portals.