Natural gas is an attractive fuel for heavy-duty internal combustion engines as it has the potential to reduce CO2, particulate, and NOx emissions. This study reports optical investigations on the effect of methane stratification at lean combustion conditions in a heavy-duty optical diesel engine converted to spark-ignition operation. The combination of the direct injector (DI) and port-fuel injectors (PFI) fueling allows different levels of in-cylinder fuel stratification. The engine was operated in skip-firing mode, and high-speed natural combustion luminosity color images were recorded using a high-speed color camera from the bottom view, along with in-cylinder pressure measurements. The results from methane combustion based on port-fuel injections indicate the lean burn limit at λ = 1.4. To improve the lean limit of methane combustion, fuel stratification is introduced into the mixture using direct injections. Two different volume fractions of direct injections (20% and 40% by volume) were tested in addition to the port-fuel injections. With stratification, the combustion phasing is shifted towards the TDC, and combustion duration decreases, indicating a faster combustion process. The stratification strategy obtained by combining the direct injection of 40 % methane by volume and port-fuel injection of 60 % methane by volume, shows highly stable combustion with COV of IMEPg as 2.85 %, rapid heat release, and shorter burn duration. The high-speed color images provide insights into the spatial and temporal evolution of flame propagation, while a HSV-model (Hue Saturation Value) analysis enables the identification of different combustion regions based on observed color changes during the combustion process.