It's possible to build decentralized order books.

Order matching does not require escrow with a third party.

Orders can be executed with atomic swaps on the blockchain.

At the same time, it's possible to use semi- or fully trusted third parties for any or all of those functions.

I expect both models to see real world usage.

But this doesn't really address the original statement you made. Originally you did not specify that you were talking about certain ways markets could be built that utilized colored coins.

Without that clarification, your statement about sidechains being "more decentralized" is false.

Colored coin tokens are stored on the main chain and enjoy the full security model of Bitcoin. The same can not be said for tokens created on a sidechain.