How are the colored coins "backed" by the gold.
Colored coins can be used to track ownership of securities, such as shares, bonds, IOUs and so on.
The guy that issued the colored "gold coins" wont send the gold to me?!
The guy who issued "gold coins" is supposed to demonstrate a signed contract and evidence that he is trustworthy and owns the asset.
It's up to you to decide if he is trustworthy enough.
In simple terms, colored coins allow you to do same things you could do on centralized Bitcoin 'stock exchanges', like BTCT.co, BitFunder.com, HavelockInvestments etc., but in a decentralized way: you don't have to trust your bitcoins to exchange.
However, trustworthiness of assets (securities) is the same, and I'm sure we'll see a lot of scams and bankruptcies.
As for gold, there were funds traded on BTCT.co, e.g.:
https://bitcointalk.org/index.php?topic=133556.0 Some guys actually posted photos of physical gold coins they own.
But I think at some point bigger and more trustworthy companies will get involved.
So this concepts makes it necessary that people trust me as the issuer?
If somebody tells you that it is possible to create gold-backed coins which require no trust, he is lying.
I know that some projects (not related to colored coins) put cryptocurrency into escrow to "guarantee" value of "gold" coins... I hope I don't need to explain why it won't work so well...