(Sorry if this is the wrong forum)

One of the most interesting topics I learned about at the Bitcoin 2013 conference was the idea of Colored Coins.  However, I'm unable to convince myself that implementing them on top of Bitcoin would have the same security characteristics that exist for Bitcoin itself.

In particular, what is the chain of reasoning that demonstrates that the set of incentives that protect Bitcoin from an economically-motivated 51% attack also apply when certain Bitcoins are colored?

More precisely, here is what I have in mind:

* For Bitcoin itself, an economically rational actor has little incentive to perform a 51% attack to steal Bitcoins (by double-spending, say) since undermining the integrity of the network would reduce/destroy the value of the very coins the attacker was seeking to steal.  Thus, the only actors one would expect to engage in a 51% attack would be those with a non-economic motive (governments, perhaps).  Perhaps another way of saying it is that the possessor of huge amounts of compute power has more to gain by participating in the system than by seeking to thwart it.

* However, if one now turns attention to colored coins, the incentives change.  These are coins with some connection to the "real world": possessing one might entitle the possessor to income from shares or title to a physical asset, etc, etc.  As such, the value of the assets represented by colored coins could be orders of magnitude greater than the value of the underlying Bitcoins themselves.   Therefore, an economically-rational attacker set on stealing real-world assets may no longer care about the value of Bitcoins (or a subsequent failure of the system) since their eyes are on a different goal.

* So... if there were ever a colored Bitcoin that was worth vastly more than the underlying Bitcoin that "carried" its color, why would an attacker not, say, acquire the colored coins for fiat, sell them to somebody else for fiat and then, once the fiat was safely received, mount a 51% attack to reverse the "sale"?   The end-result would be that the attacker had both the original fiat *and* a blockchain that recorded them as the owner of the asset.  If the (BTC) value of the double-spent Bitcoins was low (imagine 1 colored Satoshi that represents one million Apple Shares), one could imagine a scenario where the core Bitcoin network shrugged off the issue as an aberration and the attacker was now recorded as the rightful owner of a stolen asset.  Equally, one could imagine that this fatally undermines the Bitcoin system yet the attacker still potentially has title to the asset.

* Of course, the 51% attack is just an example - the wider point I'm trying to make is that it is not immediately obvious that the economic incentives that help protect Bitcoin from various attacks are still effective in the presence of highly valuable colored coins.

Am I missing something?  Is this something that one would expect to be covered in the legal agreements that "link" a real-world asset to a particular set of colored coins? Something else?