Both parties need to commit to trades at the same time. Otherwise,

1) seller offers bitcoin for coloredUSD
2) buyer accepts offer
3) seller signs his side of txn (seller can no longer back out)
4) buyer watches bitcoin price
        If bitcoin price goes up, sign txn -> profit
        If bitcoin price goes down, do not sign txn -> find new seller offering more better exchange rate.

You need to force people to move simultaneously. Otherwise the party who moves first will always take a loss. (i.e. it is like the seller is giving away a free option)