The huge yet flexible power consumption of data centers makes them promising resources for demand response, particularly for emergency demand response (EDR) which requires a certain amount of load curtailment during emergencies. However, current data centers often participate in EDR by starting up their backup diesel generators, resulting in both high costs and large carbon emissions. In this paper, we focus on cost-effective and eco-friendly demand response in colocation data centers by designing economic incentives for tenants to reduce their loads during emergency periods for EDR. In particular, we model and analyze the interaction among the data center operator and tenants by using Nash bargaining theory, and derive the optimal solutions for the load reduction and reimbursement for each tenant under two different bargaining protocols (i.e., sequential bargaining and concurrent bargaining). We prove that the derived solutions are Pareto-efficient and fair, and therefore self-enforcing and satisfactory for all entities. Numerical results based on trace-driven simulations show that the proposed bargaining approach is beneficial to both the data center operator and tenants, while also reducing the carbon emissions to the environment from data center demand response.