Addressing the powders you actually asked about:
TrailBoss was designed for large, low pressure cartridges like the .45 Colt.
As for SR-4756, it is similar in burning rate to Unique and works quite well in cartridges like the .45 Colt, although it does not fill the case as well as TrailBoss with loads limited to SAAMI peak pressures. It is usually a clean burning powder at higher pressures, but will probably be dirtier than TrailBoss at SAAMI pressures for the Colt. If you are shooting lead, it will probably not be much different in net effect than TrailBoss because most of the dirt will come from the lube on the bullets.
Neither powder is suitable for the high pressure, high velocity .45 Colt loads known as "Ruger Only" loads, but you can safely get more velocity with SR-4756 than with TrailBoss within SAAMI pressure limits.
SL1
SL1