The authors use galaxy evolution models to investigate the variations in stellar mass-to-light ratios ($M/L$) within and among galaxies. They find a strong correlation between $M/L$ and galaxy color, with the Salpeter initial mass function (IMF) over-predicting the observed $M/L$ values. A modified IMF with fewer low-mass stars fits the observations better. Applying their color-$M/L$ relation to the Tully-Fisher (TF) relation, they derive a stellar mass TF-relation that is independent of the passband used. Adding the HI gas mass, they find that the maximum slope of the baryonic TF-relation is 3.5. The study highlights the importance of considering maximum disk constraints and excluding low-luminosity dwarfs when analyzing TF relations.The authors use galaxy evolution models to investigate the variations in stellar mass-to-light ratios ($M/L$) within and among galaxies. They find a strong correlation between $M/L$ and galaxy color, with the Salpeter initial mass function (IMF) over-predicting the observed $M/L$ values. A modified IMF with fewer low-mass stars fits the observations better. Applying their color-$M/L$ relation to the Tully-Fisher (TF) relation, they derive a stellar mass TF-relation that is independent of the passband used. Adding the HI gas mass, they find that the maximum slope of the baryonic TF-relation is 3.5. The study highlights the importance of considering maximum disk constraints and excluding low-luminosity dwarfs when analyzing TF relations.