... this won't be your primary funding source right?
you can't expect to go out and make money in a field that is already filled with experienced businesses...
anyways...
pick neutral colors, blacks, reds, blues and stick with those for cases you stock.
make sure you start a website that supports custom options so people can choose to commision a variation of a case from you, or try to find ways to get them to contact you personally for totally custom quotes.
for painting...
you need to either remove all the paint (preferable) or you need to scuff it up when it comes from the factory. you can't just paint over it or it will fall off in a week
you'll need to use primers, probably self etching if you want it to stick better, and you'll need quite a few coats of that with sanding in between to get a super smooth coat.
after the primer is on, you'll need your choice of color which can be flat/matte or gloss or a mixture depending on the companies you choose. you'll spray on multiple coats, sanding in between.
after you finish painting with your main color, you'll be using clear coats not 'gloss' to protect the finish... however if you choose to use them, you'll be making your paint glossier, thus they will no longer be matte/flat. you'll do a couple coats of that.
once that is finished, if you want a professional finish, you can wet sand with 800+ grit sand paper and then buff/polish/wax
if that is too much work then you can skip the sanding in between coats and just skip the finishing portion, but the paint job will definitely look home made... it will not look factory or autobody quality at all...
my recommendation is to not paint the cases... if you plan on marketing them as high quality, you'll be spending at least a week per case...