Washing with brown is about the easiest way to get good looking orcs. The idea is that you paint neat flat colours and then paint a lot of Agrax Earthshade or similar brown wash all over the model. The blades and armours turn into nice, rusty-looking metal, the skin and clothes get shading and become dirty and orcish-looking. With that method you can start with a dark silver metal colour, brown for the bows and spears, a colour for the skin and maybe two colours for the clothes and leather bits.
When you buy brushes, get either miniature or watercolour brushes. Even though we use acrylic paints, acrylic brushes are not the best ones for this hobby. Artists' acrylics are typically a bit thicker and heavier than miniature acrylics and therefore the artists use a bit stiffer brushes. The softer brushes meant for watercolour or miniature painting work better for us.
And there's no need to fear mixing colours, it's the purpose of the paint to get mixed with others. I use cocktail sticks to get a bit of colour A on a palette (I have just a white ceramic plate, nothing fancy), another stick to get bit of colour B on top of it, and mix them together by swirling the brush around them. If the resulting colour is not good, you don't even have to paint it on the miniature.
The most typical use of mixing for me is to get a harmonious highlight colour and usually it's as simple as adding a small bit of beige to the base colour. But no need to get there too fast, start with neat flat colours and learn washing, dry-brushing or highlighting one at a time and see what you like best.
-- Pasi