A dig at the site, in Bath, uncovered remnants of red paint on the outside wall - contradicting a widely-held assumption they were white in colour.
The discovery was made during a dig in an area of the world heritage site not currently open to the public.
Manager Stephen Clews said it would have helped the building to stand out to visitors.
"Our assumption was that it was white but it's turned out to be red," he said.
"On the site, we do have examples of both red and white paint on internal surfaces but not on external ones.
"One thing we tend to forget is that the ancient world was in colour.
"Probably the simplest and cheapest solutions was to paint buildings white. So painting it a bright red would have made it more striking visually."
The dig underneath York Street and Swallow Street is part of the Archway Centre project, which is due to open to the public in 2019.
The building housing some of Britain's most famous Roman baths may have been painted red, archaeologists have said.