An analytical model has been developed to empirically study the effects of stellar spots and faculae on the observed equivalent widths of Li i 6708, Na i 5896 and K i 7699 Å lines (and abundances in the case of lithium) in late-type stars, taking into account the changes in the observed magnitudes and colors. Solar spectra corresponding to different active regions are used as input data and a range of filling factors are applied to simulate the surfaces of stars with different levels of activity. Detailed comparisons between predicted and observed photometric colors and equivalent widths are made for late-type stars of the Pleiades and the field. The observed dispersions in K i and Li i equivalent widths for Pleiades stars can be partially accounted by the simultaneous effects of activity on colors and the line formation, indicating that the lithium-rotation connection suggested for ~ 0.7-0.9 M_sun Pleiades stars could be due in part to the stellar activity. However, under realistic values for the filling factors, only a small portion of the observed spread could be explained by these effects. Based on observations collected with the Gregory-Coudé telescope, operated on the island of Tenerife by the Universitäts-Sternwarte Göttingen in the Spanish Observatorio del Teide of the Instituto de Astrofísica de Canarias.