What is it?
Acrylic is plastic. Acrylic paints are plastics, but water-soluble. Or something along those lines. They are the more manageable alternative to oil paints for artists - can be layered and mixed, but being water soluble they are a lot less messy and don't need turps to wash your brushes.
That said, they will not come out of clothes. Ever. Believe me and my poor ruined jeans.
Why are they useful?
They come in a huge range of colors, and will blend extremely well with foamed (or otherwise) gelatine. This helps get a decent base colour for your prosthetics, as opposed to the yellow that the gelatine is naturally.
Where do i get them?
Any art shop at all will have acrylic paints, but also many small shops and variety stores carry small $1 bottles. 2 or three of these in skintones, plus some black and red, will usually satisfy all your zombie needs
Many accounts tell of colouring gelatine with 'vinyl medium' aka fingerpaints. This may well work, but I don't profess to know what they are exactly, and I have not tried this method.