Hello, Arbitres here. Here to answer your Lost Colors questions!
1. Is there a route for C.C.?
Answer: Geass route is the very first route your presented with. Geass route has three endings: C.C., Lelouch, and Nunnally. You can't unlock any other routes until you do it - besides it's automatic. So your stuck with the route you want.
2. Is it worth buying?
Answer: Depends on whether or not you want the following.
1. The ability to change R1's storyline, character's fates and perspectives.
2. To fall in love with the character of your choosing to get your ending.
3. To form the character's personality however you want through choices.
4. to get your own personal knightmare frames. Kuro no Kishidan route gets the Pre-production Gekka. Britannia military gets Lancelot Club.
It depends on what you want in a video game. This is a visual novel, so it's choice-based and pretty much dialogue. There is anime cutscenes from the anime, though.
The abttles you fight in are either automatic or very choice-decisive. One mistake in a choice could cost you the battle. This also goes for certain events -- like the geass route has a particular event that could cause a game over.
I think you would enjoy interacting with your favorite characters, I know I do. I've had the game for months now, and I'm still having trouble clearing Britannia route. Just because it's a Visual Novel doesn't mean it's an easy game.
PSP edition includes - Japanese Liberation Front route exclusive. You join the JLF (it's a drop off event from kuro no Kishidan.) and either become a knightmare pilot or a Holy Sword. The endings you can get for JLF is Kallen ending and Nagisa ending.
PS2 edition includes - Blue Moon route exclusive. You become an ashford student (...Drop off of Ashford route.) with the ability to make anyone fall in love with you. Every character in Ashford has an ending -- all the important characters anyways.
There you go, and I hope that helps. =) Ask me some more questions! xD
EDIT: Some pictures to give you a taste of what to actually expect from Lost Colors. They are pretty bad quality, so I hope you don't let that be mistaken for ingame quality.