I've spent a fair amount of time looking for clean burning powders. In GENERAL, IMR and Alliant TEND to be dirty. They work fine, and in some applications they're cleaner than in others. Sometimes the firmness of the crimp makes a difference.
My searching has been in .38/.357, so I can't help you with the .45 Colt....sorry.
So far, two brands have stood out. Hodgdon (
http://data.hodgdon.com/main_menu.asp) has some that are very clean. I like Universal in particular. I've used it for training classes where hundreds of rounds were expended every day, and there were no problems with gunk building up in the gun. This does require using jacketed/semi-jacketed bullets rather than cast bullets, though. Clays is very clean, and I like it with .38 spl loads, as it is pretty fast burning. Universal is a bit slower, but too fast for high performance .357 mag loads. In .357, Universal is excellent for mid-range loads, and up to decent SD loads, which are below max. This is fine for me as I don't load max loads anyway. HS-6 with a tight crimp is fairly clean in .357 mag, but not as clean as the other two I mentioned. It does give higher performance, as it is slower burning than Universal and Clays. I use this a lot, anyway.
The other powder is Vihta Vouri. I'm just getting into this brand, and I must say it is pricey by comparison. Try N-340 for .357 mag. as a starting powder. This stuff almost cleans your gun for you. I'm still checking out others, but the reputation of this company for clean burning powders is pretty well established, judging by reloading publication reviews and comments by reloaders. I'm considering trying N-110 for rifle loads, but I'm starting to wind down my shooting and am selling off and giving away my guns, so I don't know how far I will go with the reloading. I'm sorry, but I lost my address for the V-V information site. If you cannot find what you need, PM me with your e-mail address, and I'll send you the V-V reloading manual in Adobe format.
twb