Most astronomers look at the red sequence on a scatterplot, with each point being based on the measured fluxes of one galaxy. Very few astronomers are literally looking at color images to eyeball these measurements.

There are edge cases in which astronomers will load up images at multiple wavelengths and overlay them, but this is not the normal case. By and large, they're looking at a single channel at a time. Even more commonly than that, they're working with catalogs automatically generated from images.