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Hi Dana,
Thanks for the question. I think I may have confused things a little ... By different colour-bands, I mean observing at different wavelengths using filters.
You can view a galaxy (or a star for that matter) through different filters, e.g. blue, red, infrared, etc. Different wavelengths of light are impacted by the interstellar medium and galactic dust in different ways. Blue light for example is dimmed more than red, and infrared light. We can correct for it pretty well but it's not perfect.
Because of that, magnitudes of galaxies on the Tully-Fisher line show different levels of scatter (distances from the line) for different wavelengths. Blue and Red show more scatter than near-infrared measurements. So longer wavelengths in the infrared range seem to show a stronger/closer relation to the Tully-Fisher line.
Hope this helps.
Cheers,
Af.
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