Back before the days of plastic locomotives in a plethora of paint schemes, it was normal for an MR to paint his locomotives (or have them painted, if he didn't feel up to the task). For those who built from metal kits or totally from scratch, painting was an expected part of the experience.
I hand painted a red cab roof on one of my plastic steamers as my first experiment.
Normally, the shell is stripped of existing paint and decals first. This reduces paint build-up that would obscure fine details. Kato shell use a much more sensitive plastic than most, so be careful what stripper you use.
An airbrush is the preferred way to paint a model. It is easier to get a very fine thin coat with an airbrush than with a paint brush. But I'm getting ahead of myself. After stripping, the model needs to be washed and dried to clean off any residues and oils. Only handle with gloves after this point in the process. Primer is highly recommended to make the color top coats look right. Decide what order you are going to paint the various colors/areas in and mask accordingly. Then you can paint. A glossy surface is needed for decal application. After decaling, Dull-Cote is used to make the paint flat again.
That's the basics. Costs vary widely depending on paint used (usually way less than $10 per model), decals needed, and whether you count the cost of tools (like the airbrush and compressor).
Nowadays, I figure adding $20 (usually comes out less) for trucks, couplers, extra details, paint, and decals for a craftsman model kit. Your mileage may vary. On a steam locomotive rebuild/kitbash, include remotoring and regearing, DCC and sound, and lots of extra/new details, the total can get up over $200 for everything used. But there's no other way to get an accurate model of a specific prototype. On the other hand, I've customized cheap Tyco steam locomotives for less than $50.
yours in having fun