There are toy customizer commission services out there, but the good ones are very expensive. I personally would recommend teaching yourself to customize figures yourself.
A few words of advice:
Practice on cheap figures, junkers or KOs. You don't want to ruin that lovely Leader class Movie Brawl when you could practice on a cheapo knockoff.
Invest in a good airbrush and learn to use it. Seriously, if you want to paint your figures this is the best way of doing so. Some details will need hand painting obviously, but for the vast majority of colour work you'll want to airbrush it. You'll get smoother, nicer and more durable results.
If you want to have a figure you can repose and transform, remember- less is more. The less complex the paintjob, the less likely it is to get damaged. If you want to do a CW Skydive into a Starscream for your regular CHUG display, it's best to keep the complexity of the paintjob fairly minimal. For one, whilst a Devastation inspired cel-shaded look might look cool, it'll be quite fragile due to all the layers of paint needed. Second, it'll stick out like a sore thumb next to your other figures.
ALWAYS prime your parts. Always. The paint will stick better and be more durable as a result. Unprimed plastic may cause the paint to flake.
Try and find the version of the mold you want to customize with the closest colours to what you want. That way you can leave bits of it unpainted without it looking crap, increasing durability and minimizing work.
And finally, for further advice on painting and modelling, it's a good idea to consult Gunpla forums. They usually use similar techniques to customizers, as their subject matters are, well, giant robots made of plastic. Scale modelling websites could be good as well for more advanced pieces.
Hope that helps!
