I thought this was an interesting question, and so I put on my Art Historian Hat and had a look. I also had a vague notion/memory in mind of what you are saying, but I'm having trouble finding actual examples as you describe it. Red was a very common color of choice for wall decoration, especially on plaster/stucco, but not necessarily on the bottom of walls. This is the most obvious example I could find, from a Pompeii shopfront. Also this from a Pompeii interior. But red was also common on upper courses, as here in a Pompeii dwelling. Another.

All of these show elements of a typical Roman wall-painting technique called the "First Style", which sought to imitate an expensive marble façade with plaster and paint. It is the oldest of the styles we can detect (mostly from evidence at Pompeii and Ostia), from the 3rd century BCE, and involves (in general) a basic blocking out of a flat undecorated surface into faux architectural elements. In the case of our Pompeii store-front, the red lower course simply imitates a more expensive alternative, like reddish marble façade or even terracotta facing, as the earlier Etruscans had often done. Basic blocking of elements with paint was also common in earlier Etruscan wall painting, which Romans of course could witness and imitate later (example). It should also be noted that painting was generally the cheaper, less nice version of more advanced masonry decoration, of which the Romans were experts. There were several common masonry patterns, with this one serving as a basic example: a lower course of opus testaceum (red-brick facing) with a lighter blocking of opus reticulatum (diamond-shaped) in tufa facing above it.

There are some arguments to be made that the glazing/painting of stucco was preferable to plain white, especially in high-traffic areas (to resist accumulation of grime of thousands of touches, or to discourage graffiti, etc), but I would not imagine that was ever the primary impetus. It is just basic style preference in breaking up a broad monochrome surface with blocking.