Well, you can just get all the primary spray body face colors and just mix hairstyles brides defence all the rest yourself but if you dont know color theory this could turn out badly...
As far as paints go, I HIGHLY reccomend sherwin williams P3 elephants behr paints wedding photographer from Privateer Press. The New asian fanatics fanfics part time bookie soft passport water sports village music festival 2011 food channel paints shades is smooth and coats very well and they have some of the best colors I've found. I especially like how they have a base shade tool sai my love ball marker catalog for tiles cratch shop and then one or two highlight shades off the base color. Very handy. They also sell their paints in moxsets of purple blue pink orange grey red brown green 4u 6 or so paints with a theme/Warmachine/Hordes faction in mind so you can buy groups of colors together. Cost is about the same as GW but you get a LOT more paint in the bottle than the GW paint pots... The P3 pots are far superior to the GW pots and will not dry out as fast as the GW ones will. I at one time had the full run of Citadel colors but once P3 launched I dropped GW like a rotten egg... The only GW I use now are their washes.
A decent mid Grey. Because adding black to things will often make them too dark, and grey is always a part of weapons and metal.
At least two browns, one yellowy and one red-y. The thing about the world is its full of browns - brown leather, brown cloth, brown wood. Browns are absolutely vital and you will use them like crazy. Having a yellow or red or blue to add to them will let you make different shades.
If you like to base, you'll also want a nice light green for grass.
And then a skin colour. One light and one heavy if you like, to avoid having to mix every time you do skin - assuming you're doing lots of skin. If you're doing robots it's less of an issue.
One metallic silver. Again, metal is fricking everywhere, like wood, and gold won't be quite enough for variety.
Finally, I like to get some colours for inspiration purposes, like a colour you would have no idea how to mix and just leaps out as being awesome. That will help motivate you to paint.
Why write something when someone has already done it for me? This advice above is great. I find it hard to paint when I run out of any of the above, right now I am low on a brown and it has been holding me up from starting on my Northern stuff for Heavy Gear.
But all the stuff listed above is a great primer. Once you have all those basics you can be pretty confident that all you need to do is pick the primary colours of what you want to paint and then go for it.
Also keep in mind, when picking skin tones, what culture you are painting. Though having a general skin tone is always a good idea if you are doing any African or Asian skin tones be sure to do a bit of research first. When I first started collecting Yu Jing (pan-Asian) for Infinity I had to learn to paint fleshtones for them and finding the right colours was pretty difficult (love Vallejo for having a perfect set of colours for Asian skin tones). So if your first project isn't full of Caucasians then you will likely want to skip "standard" skin tones right away and get the ones right for the people you are painting.