The .45 Colt is my favorite round, and I own several. I have been loading this round for about 21 years, in several different types of revo's, from the 25 platform's all the way to the Redhawk, which is on a whole different plane.
I regularly use 9 grains of Unique under a 255 cast swc, Fed 150 primers, WW cases and I am getting 865 fps across the Oehler 35P.
I use 8 grains of 231, with the same components for about the same velocity.
These loads were fired out of my 4" 25-5, which has the same barrel length as yours. Both loads are very good loads, and shoot well out of all my 25's.
12.5 grains of HS-6 gives me right at 1,000 fps with the same components, and is running right around the same pressure as the Unique load according to the source I got the loads from.
18.5 grains of 2400 will show about 1100 across the chrono with the same components.