Shades of Blue - Hume

The shades of Blue argument that Hume proposes seems like he goes against his own argument that he must have experienced something previously...and he doesn't care! What kind of philosopher is this guy, he pokes holes in his own beliefs and then walks away saying, 'Eh, that's not important'! If everything you have is supposed to have come from your impressions of the natural world, why can you sense the lacking of a color, taste, or musical note from a given scale of something that you haven't seen/heard/tasted before?