The black roofs and the darker gray colors were introduced at different times, Chuck. The roofs on first two sets of passenger cars converted in 1947 for the Indianapolis trains were gray to match the new F3's but the the next two sets converted in 1948 for the Louisville trains appeared with black roofs after management apparently had decided that having gray roofs was impractical, given diesel exhaust soot and that most other railroads still were under steam at the time. So it wasn't long after that the locomotives were repainted to match.
On the other hand, all the passenger equipment, cars and locomotives, retained their original two-tone gray until around 1954, as shown in this 1954 photo of one of the 20-series coaches in its original paint, followed by a newly repainted
single-tone-gray Diner-Parlor-Observation, either #58 or 59.
