Substitution of nature's services with technological alternatives has been pursued as societies have industrialized over the past three centuries. But the time for reverse substitution may be upon us. In a wide variety of settings, from water purification to climate change adaptation, investors are increasingly considering the worthiness of green infrastructure solutions, such as mangrove restoration, rather than conventional gray investments, such as sea walls, to achieve the same environmental quality outcomes. The problem is that infrastructure investors do not have a consistent and robust way to compare gray with green infrastructure in an apples-to-apples manner that is convincing to budget hawks. In addition, uncertainty is greater with 'unproven' green infrastructure approaches. As a result, green solutions are often neglected. Here, we present the contours of a general methodology called green-gray analysis (GGA) and demonstrate its usefulness in a green-gray trade-off facing the Portland Water District in Maine. Results provide evidence for the superiority of green investments in several scenarios, purely on financial terms. When ancillary benefits, such as carbon sequestration or passive-use values for Atlantic salmon are factored in, the case becomes even more compelling.