When you are smoking weed, the head change will occur when you first feel the effects of marijuana. You are not high yet, but you're getting there. Usually the effect of smoking too little weed will result in a head change. A slight difference, but you're not high\, yet. The head change effect will wear off fast and result in wanting some more herb.
-there were too many heads on that blunt.
-yeah, i got a head change, that's it.
When someone has purps. Purple fuckin weed.
Perhaps they are selling them, maybe they're just braggin. ignore that idiot that said it means extremely potent weed. There many kinds of potent weed. Purple, well that's just different, a whole nother level.
guy with the grapes- hey you smokin'?
guy without the grapes- yeah...some green buds.
guy w/grapes- I got grapes!