Guan, I would recommend using aluminum powder over the nickel/silver that smooth-on sells, because the nickel is super expensive. $50 a pound? Hooooly ****. Depending on how heavy it is (probably quite) then that 1lb may not go very far either. So far I've used aluminum, bronze and copper, and 1lb of aluminum is significantly more powder than 1lb of bronze. www.sculpturedepot.net sells aluminum powder that I've used before, with good results with the smooth cast 325.
I don't know that this is exactly a substitute for painting either. It will looks silvery, yes, but that's it. It's still a blank slate that you need to weather over the top of, it's simply with cold casting, the technique is different. You can use paint if you want, I personally use a propane torch and aluma black.
Elkman, I believe the smooth cast 300 doesn't take pigment quite as well as some of the others, 320 and 325 especially, which might account for the lack of effect you got.
In case it helps, here's some pics I took for someone while I was doing a cold cast celtic bio. I do it a little different to the smooth on video.
Measure everything out
Pour part b in the cup and add tint
Add metal powder and mix
It'll look a little sludgey till you add the part a (not pictured)
Part a has been mixed in, and everything poured into the mold
Slush until it starts to set