We report a simple post-synthetic approach involving a brief high-temperature treatment under vacuum that effectively turns the typically yellow PCN into a red PCN by extending significantly its visible light absorption to >600 nm. Most importantly, we provide evidence that the extended optical absorption edge of PCNs subjected to high-temperature treatment is due to the formation of azo-moieties that act as linkages between the heptazine units inherent to PCN structure and/or between heptazine and triazine units formed upon partial fragmentation of PCN. Our present understanding is that the formation of planar-R-CNx units comprising the azo-linkages and embedded in the yellow carbon nitride framework is responsible for the reduction of the overall bandgap and the red color of the material. Notably, the azo moieties effectively sensitize the red PCN for photocatalytic H2 evolution even under low energy (470 nm) light where conventional yellow PCN is inactive. Moreover, the red carbon nitride clearly contains moieties that can act as metal-free catalytic sites for H2 as evidenced by effective operation even without any additional co-catalyst. The downside of the high-temperature treatment of PCN is the partial fragmentation of the heptazine network creating large number of recombination centers and decreasing the overall photocatalytic activity of the resulting photocatalyst. We assume that our findings on the so far overlooked role of azo moieties in PCNs with strongly extended absorption edge might not only be valid only for this particular case, but also for other cases in which a strong shift of the PCN absorption edge has been observed upon thermal treatment procedures at temperatures exceeding the temperature of ammonia decomposition into N2 and H2, rendering thus the atmosphere more reductive. The present work thus highlights the importance of a careful determination of structural features governing the complex interplay between the light absorption, charge separation and catalytic turnover in PCN-based polymers tailored for visible light-driven photocatalysis.