I see 2 questions here.
1) Do you present a digital mockup or a printed one?
2) How do I control the diferences in color between the thing I see on screen and the final printed version?
For the first one. Depends on which part of the process you are and what kind of package you are talking about. Is it just a sticker on a already manufactured flask, metal box designs where the actual provider of the boxes have specific sizes, or a custom/bespoke cardboard box you are designing from scratch?
I sometimes present some rough isometric views in the first stages, or a digital 3D model, then only the design in 2D, then a 3d real life model... Depends.
But in the second topic about color, it is not RGB vs. CMYK, the problem you have is that you must have a colour calibrated workflow.
You can present a 3D model or print some samples, but if all the process is not calibrated you will have a lot of diferences eaven with your printed samples ane the final printed product.
A pantone color guide can assist you, and you can define plain colors based in it, but if the design includes photo design you need to calibrate your monitor and printers with something like this http://www.colormunki.com/.