All value is created by humans, and value comes from belief, its the same for Bitcoin as it is for the US dollar, and the same for gold and silver.

So 'backing' something is just using the stronger belief in one thing to lend belief to another thing. In a sense this is what 'coloured coins' does, you write an IOU into a Satoshi and store it on the Bitcoin ledger, you can write anything on that IOU, like I owe you $1,000 or I owe you as specific house, Bitcoin is just a ledger. However in this use case, the actual price of a Satoshi is irrelevant, its just a way to use the Bitcoin ledger.

I dont have deep knowledge of coloured coins, but its worth a look.