This paper asks a simple but important question: when does integration into green global value chains actually reduce a country's emissions? Using clean manufacturing sectors identified by the International Energy Agency, we build new forward and backward measures of green GVC participation for G20 countries from 2001 to 2022. Using within-country changes and post-2017 Paris Agreement triple-interaction design, we study state-contingent effects of green GVC participation on emissions based on two key factors: renewable energy share and energy intensity. The findings show that green GVC integration is not automatically good for the climate. For forward participation, emissions fall most when renewable energy use is low. As the renewable share rises above 6.5%, this benefit weakens and eventually turns positive. Higher energy intensity also constrains the gains and causes reversal at 4.5 MJ per unit of GDP. Backward participation shows weaker and more asymmetric effects. The Paris Agreement brought a clear regime shift that weakens the energy-intensity constraint and dynamically reconfigures renewable-contingent effects. Overall, green GVCs are not automatically green. Trade-based climate strategies deliver real environmental gains only when supported by progress in renewable energy and energy efficiency.