I've used many powders that was dull. Ive been told it's when the powder is dusty. I've even used the old tin can imr3031 when it had a very slight rusty dust. But dont take my advice. How much is very slight ? How did it work? A SD of 9 in a 30-30win model 94.
You need to be careful with that rust colored powder if it has a slightly acidic smell.
That acid can do harm to a bore. I had a lovely Model 70 chambered for the 257Roberts and used some #44 Nobel powder, which I got from Higginsons.
The powder had some dust that looked like rust in it, which came from the top and bottom of the container. The walls were paper.
I loaded up enough cartridges for a weekend match and because the rifle was shooting extremely well and I only needed ten more rounds to finish the next morning, I didn't clean the bore.
By the next morning, approximately 24 hours later, there was a fine film of rust through the entire bore and in the neck area of the chamber.
I ran a patch through it and the rust was very evident. The bore looked OK so I shot it and it still shot well.
I didn't clean it again, because I thought the rust was coming from the humid conditions (duh) until the next morning and I had a long drive back home.
Same thing, fine rust. The damage wasn't evident with an eyeball but later on, with an endoscope the pitting was quite obvious.
I shot that barrel for another year but ended up dumping all of the #44 powder (IMR 3031 equivalent) as every can I opened had the same rust powder and a slightly acidic smell.