Operando color microscopy and ex situ AFM were used to investigate the lithiation process in pure (a-Si:H) and methylated (a-Si1-x(CH3)x:H) amorphous silicon thin layers. Color analysis of optical images allows for monitoring thickness changes of a-Si:H layers. Unlike pure a-Si:H, the first lithiation of a-Si1-x(CH3)x:H is found to be spatially non-uniform: lithiation starts at a limited number of locations then expands radially, forming circular lithiation spots. The morphology of the lithiation spots and their evolution is accurately measured by ex situ AFM. A mechanism is proposed to explain this phenomenon, involving the high resistivity of methylated silicon and the existence of low-resistance point defects.