The Army Painter spray primers are ideal for this, and you can get the exact matching paint to go over any missed bits.
They do two browns - Leather Brown and Fur Brown. Necrotic Flrsh may also be suitable, depending on what colour you want your Orcs' skin.
I generally use Leather Brown as a primer for most of my figures, so I can leave some parts (boots, bags, belts, etc, some horses) in the primer colour and not need to paint them with another colour.
I'm sure you can get some painting guides (on the Army Painter website or on Youtube) on painting with this system - spray primer in the most common colour, paint the other parts, each with a single colour, finish with AP quick shade).