It's the same as with L4D. Just make sure you set "sv_cheats 1" to a button. Having sv_cheats set to 1 by default doesn't work in Portal or any Orange Box games for some reason. Only L4D.
Here's a parameters.ini, then. I've tested the sphere colors from 0 to 6: 0 - White 1 - Grey 2 - Black 3 - Peach 4 - Yellow 5 - Lime green 6 - Sky blue I'm sure there are more after that, but I can't be bothered to test more at the moment.
Don't forget to include the new parameters.ini so that you can change the color of the foreground and avatar lighting. The black globe is much nicer than the default one.
It will be in the public folder (0000000000000000). If you're using XPort 360 it'll tell you the names of the games on the folder, but if you're using Xplorer360 you'll have to know the ID in advance (or else guess and check).