That's not brass that's partially burnt powder; the yellow color is a typical characteristic of that. It's not really "unburnt", the powder doesn't look like that before it burns, but it's not completely burnt either.
It's a typical symptom of low powered subsonic ammo.
I second the question about why subs. Too many people are under the mistaken assumption that subs are somehow better, or somehow safer in crowded areas. Both assumptions are wrong; the only thing better about subs is the reduced noise. The likelihood of extreme over-penetration is very high as well, much more so than if you used a good 110-125gr high velocity round.