I would not recommend buying any color sets. Especially not from GW!
In order to choose the "right" paints, try to decide roughly on a colour scheme you want, and then ask yourself what colors you need for this colour scheme. Bear in mind that you should (usually) pick 2 shades for each color you want to use: one BASE color and one to lighten it up. Buying Intermediate shades make your life easier, because you won't have to mix as much, but are not compulsory.
What also helps is if you think of the textures you will need to paint, and the colours they will require: do my minis wear a lot of leather? Have huge teeth and horns? etc. This helps you decide.
You decide your mercenaries should be green/white.
You will probably need:
1) a dark green and a light green to paint the green parts.
2) a grey (or brown) and a white (or beige) depending on the type of white you prefer.
NB: some people like to lighten everything up with white. I personally don't think that looks good with every color. Red, for instance, does not only get lighter with white but also pinker. And I usually don't want that effect.
Aditionally to those individual colours you need, I would say the absolute essentials that should form the base of any painting/modelling set are as follows:
- a base for flesh: I use a snakebite-ish colour (light brown)
- a flesh colour
- a dark metallic
- a light metallic colour
(however some people prefer to paint non-metallic weapons)
- a dark brown
- a white
- a black
- a small drill
- a file
- a modelling knife (doesnt need to be the GW one!)
- PVA Glue
- Flock/Sand (both recommended)
I only use black primer. And I think so do 90% of the people. But I know that some really good painters use white. You'll just have to try what works best for you.
However I recommend black primer.
With around 10+ paints you should have a solid base to paint.
Oh, and do yourself a favour... buy as much of that from Vallejo as you can
PS: I'll post a list of my personal paint set later if you wish. Maybe that will give you an idea.