How are colored coins enforced? How are colored coins enforced?

A signature can easily be rejected, and so can a block.

How does the blockchain enforce that a particular asset is tied to the colored coin or has been delivered?

I can see how another cryptosecurity could be tied to a cryptocurrency, but does the blockchain physically manifest itself, kick down the door of 31uEbMgunupShBVTewXjtqbBv5MndwfXhb, and collect the asset if 31... doesn't deliver?
You could imagine that you're selling a car to someone. The key to open the car is tied to your bitcoin address. So only you, owner of the private key for that address, can open and turn on the car.

Once you sell the car you make a symbolical transaction to the buyer and you configure the car key with the buyer's address, so only s/he can open the car.

The symbolical transaction is actually a colored coin. It's proof that you've sold the car. That proof is also attached to the physical key itself. That proof will forever exist (or at least until bitcoin exists) in the blockchain.

If then they decide to sell the car once more, they can use that same colored coin and make another symbolical transaction. Now the third owner of the car can see the proof of ownership of his car just by looking at the blockchain.