I don't shoot much full length 577/450 anymore. Not that I don't like it but my eyes are't what they used to be, so now I shoot 45 colt cartridges in inserts. Still use a grain of Unique under a charge of compressed 3F black powder.
The load I used for the full size round was 6 grains of Unique under 60 grains of 3F black powder. Not a hot load but very close to what the OP is using but with faster burning powder.
The reason I went to faster burning black powder with a smokeless powder starter was to get a better/cleaner burn of the propellant. An old friend/smith and black powder aficionado, Jack Stead from Salmon Arm gave me this recipe about two decades ago. I use a similar recipe in my Snider. I use paper patched bullets, when I have been arduous enough to wrap my bullets. Mostly, I just use as cast bullets that I powder coat. My rifles don't seem to know there should be a difference. The only thing I use for the hollow base bullets is Bondo. I just fill in the base with as much as it will hold and let it harden overnight. Why mess around with precast fillers??
The only other extra I add is a cardboard spacer between the bullet base and the powder column. I'm not sure this is even needed with powder coated bullets. No, I don't worry about what color powder coat to use.
This system works well for me and YES, it's safe. It also eliminates a lot of tedious steps while delivering very good results