The main problem of smokeless powder vs. black powder is the Peak Pressure upon ignition. Black Powder keeps a "more" even pressure as it ignites and "explodes" where as smokeless powder hits a very high pressure just as ignition happens. This peak pressure is much higher than an old BP revolver can stand. Even the Cowboy light loads are waaay to much for an old revolver.
The other part of shooting smokeless is we have no idea what the revolver has been thru in it's past years. Many of them probably had smokeless fired thru them when smokeless came out. This could have cause some very slight fractures in the old malleable iron frames that you can't see. It could let go at anytime and a nice Colt would be destroyed and the shooter possibly hurt.
I know that the S/N of around 190000 is the recommended number that smokeless (light loads) powder can be fired, but I ere on the side of caution and don't shoot any smokeless untill the S/N is above 225000. I may be too cautious, but I want to keep my fingers and destroy a fine Colt.