Is a homeowner obligated financially to a subcontractor when all dealings have been with a now unresponsive contractor?

We live in Colorado. We hired a contractor to perform landscaping work on our backyard. Work was started by the contractor's crew but not finished. A sub-contractor was brought in to complete a concrete patio which was completed today. The contractor has been unreachable by email, phone, etc. for several days. Unsuccessful attempts have been made by myself and the sub-contractor. The sub-contractor now is threatening to file a mechanic's lien because the contractor has not paid him. We initially paid the contractor one half payment of the work to be performed (including the concrete patio). The sub-contractor has proposed that we pay him directly to avoid the filing of the lien. Isn't our contract commitment solely through the contractor? Shouldn't the sub-contractor be pursuing legalities through the contractor? What are our rights/obligations to the sub-contractor? Does the sub-contractor have the right to ask us for payment directly?