Colorado air-quality regulators this week will tackle one of the more complicated rules the state has drafted, ordering about 40 natural gas companies to reduce their greenhouse-gas emissions or pay into a system designed to help businesses lower their carbon output. The Colorado Air Quality Control Commission is set to create the new regulation during what’s expected to be a grueling three-day hearing starting Wednesday, at which air regulators will try to follow state law, preserve the state’s oil and gas economy and satisfy environmentalists who want to slow climate change and improve public health. “This rule is very, very hard,” said Christy Woodward, regulatory affairs adviser for the Colorado Oil and Gas Association.