The color I use most is a warm, dark brown called Walnut Brown. I use it for lining, for painting eyes, under metals, and a lot of places where black would be just a bit too dark. I've probably used a bit of it on nearly every miniature I've ever painted, and I like to use it as a measure for how long the 1/2 ounce bottles last.
A 1/2 ounce bottle will keep for about 3-4 years with normal use, and it's enough to paint roughly 200 miniatures. Once you discover how much paint you need for coverage, you will hardly use any.
I use six different brands of paint, each has its own strengths and weaknesses. The Reaper paints are great for wet blending and washes because they have a lot of flow improver. Their reds and yellows are poor, because the amount of flow improver makes it almost completely transparent. The reds and yellows in the HD paint line are better, but personally I would go with a different brand for those colors.
To save money, some folks use craft paints, stuff like Apple Barrel that you can get for $1 a tube at Walmart and Hobby Lobby. Those are cheap paints, however, and you may have problems with gunk from thick pigment, thick medium that obscures details on the miniature, or pigment flaking off when dry. It's an economical way to start.
I have a lot of paints, but you don't actually need more than a few. Meg Maples has won awards for her miniatures, and she doesn't use but 5 paints. Every color she uses, she mixes from those.