I can't really say if it's the color, or some other different characteristics of the different powders, but every color I've used acts a little differently. They all work, although some like the Harbor Freight stuff are pretty poor, but that particular green is the best I've tried. It coats nicely, with a slick finish, and doesn't pull too much coating off the bullets when breaking them apart.
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I have an answer on this point, from a PhD in Chemistry no less. My buddy is getting into PC and bullet casting for his 458 SOCOM. I showed him this thread. He's using the Harbor Freight sprayer, and I thought this might be easier for him.
He pondered the idea of how the different colors work, and said it made sense. He said that if you assume the only difference is the coloring agent, then the difference is down at the molecular level, and different sized molecules would either impede or promote bonding with the lead alloy.
The grass green paint probably has Titanium as a major constituent of the colorant. I suppose there is something to be said for coating your bullets in Titanium, huh?
BTW: My buddy is now experimenting with Krylon spray paint as an alternative.
