Unlike oils, acrylics can also be used on paper because they do not cause decay in natural materials. Acrylics are pigment suspended in a polymer binder which can be dissolved in water before they dry. After the surface of the paint has begun to congeal, the areas which are dry to the touch are permanent and can no longer be dissolved. Clean up tools and palette with soap and water. Always use synthetic bristle brushes with acrylics.
Nylon brushes work well.
Natural bristles will draw in the moisture which causes them to swell, then break down. Color is something that can be frustrating to deal with in acrylic painting. The polymer binder in acrylic paint has a semi-opaque milky look before it is dry. As the paint dries the paint becomes transparent. This taints the way one sees the color.