This approach does still NOT deal with the issue of trust or fraud - while it would be impossible to counterfeit a certain Bitcoin, it might be possible, that the car does not even exist any more or the one who claimed to hand out the car actually doesn't want to sell the car... Same goes for anything else that can be tracked in the block chain. I could hand out 1 million coloured coins for 1 EUR each and sell them for bitcoins. If the price of BTC rises against the EUR it might even be possible to pull this stunt off, so for the extreme bulls out there, here's how to do leverage in the wild west!
Anyways, just like one has to trust me that I'll be able and willing to pay back EUR for the coins I handed out, there's also no way to force me to do that - and good luck in court with something like THAT ("I paid internet funny money for some different internet funny money, but this guy said he'd give me EUR for that different internet funny money that is actually the same internet funny money as the first one. - No, he's NOT a licensed e-money dealer at all, I just fired up this software...").
I prefer Ripple to this one, as it deals with this trust issue by involving people you know (as far as I understood it) instead of staying completely pseudonymous.
You've hit the nail on the head - that is the problem with colored coins - you have to trust the issuer to support their value.
This problem is exactly what I wrote my paper about!