If I am unsatisfied with my artist's work before a multi session piece is complete, what should I do?

The artist has an impressive portfolio, which I checked before starting. He drafted a nice sketch based on my idea, and the line work turned out great. However, now that we are 80% complete, the shading/coloring seems sub par. I'm not sure if it's best to voice my complaints now to see what can be done to fix them, or if I should wait until the tattoo is entirely finished before worrying about it. This is my first multi-session/detailed/colored tattoo, so I'm not sure how much is supposed to come in towards the end.

I'm leaning towards waiting it out (he's done this before, must know what he's doing, etc. etc), but I can't help but feel nervous. Any advice or reassurances would be welcome.

Edit: Hey guys! Thanks for all the advice, I really appreciate it. I think I am just going to go talk to the artist before our next session and let him know my concerns. I'm sure it'll be fine. I know a few of you have requested pictures, (and I understand how it would be helpful!) but I'm sadly unable to provide them right now. When I get it finished, and assuming it looks as good as I'm hoping, I'll post some before/afters. Thanks again!