Demand response of datacenters recently has received increasing efforts due to huge demands and flexible power control knobs. However, very few works focus on a critical segment of datacenter business: multi-tenant colocation. In colocation datacenters, while there exist multiple tenants who manger their own servers, the colocation operator only provides other facilities such as cooling, reliable power, and network connectivity. Therefore, colocation has a unique challenge for the demand response: uncoordinated power management among tenants. To tackle this challenge, we study how to coordinate tentants for economic demand response. We show that there is an interaction between the operator and tenants' strategies, where each side maximizes its own benefit. Hence, we apply a two-stage Stackelberg game to analyze this scenario and derive this game's equilibria. Finally, trace-based simulations are also provided to illustrate the efficacy of our proposed incentive schemes.