If you don't do much touch up, just continue buying from dealerships and body shops. But @ $20 per bottle, that's going to add up! Plus you can't tint the paint if it's not a perfect match. Plus the road time spent shagging the paint.
As for light metallics...they never come out perfect. Doesn't matter who mixed the paint, who applied, who added "metallic suspenders" and used squeegees, airbrushes, paint thickener...doesn't matter. Just do your best and make sure the customer understands the improvement will be better, but not amazing.
Dark colors (metallic or not), solid colors (red, white) come out great, and you should always offer touch up of chips, scratches, and door edges.
There's 3 main kit makers:
Applied Colors ($1095)
Rightlook ($1995)
Detail King ($1695)
All of these kits allow you to tint your paint up and down until you get a match.
I've done lots of touch up. In time, you learn not to be a perfectionist because it just isn't possible. Just tell the customer you'll recolor the chips/scratches and they'll look a lot better from normal distances. Just like every reconditioning service, it's all about selling it right.