We compare schematic modeling of spots and plage on the surface of cool dwarfs with Pleiades data to assess effects of magnetic activity on the strengths of the LiI and KI resonance lines in Pleiades spectra. Comprehensive LiI and KI NLTE line formation computation is combined with comparatively well-established empirical solar spot and plage stratifications for solar-like stars. For other stars, we use theoretical constructs to model spots and plage that portray recipes commonly applied in stellar activity analyses. We find that - up to B-V=~1.1 - neither the LiI 670.8nm nor the KI 769.9nm line is sensitive to the presence of a chromosphere, in contrast to what is often supposed. Instead, both lines respond to the effects of activity on the stratification in the deep photosphere. They do so in similar fashion, making the KI line a valid proxy to study LiI line formation without spread from abundance variations. The computed effects of activity on line strength are opposite between plage and spots, differ noticeably between the empirical and theoretical solar-like stratifications, and considerably affect stellar broad-band colors. Our results indicate that one can neither easily establish, nor easily exclude, magnetic activity as major provider of KI line strength variation in the Pleiades. Since LiI line formation follows KI line formation closely, the same holds for LiI and the apparent lithium abundance.