At the most basic level, acrylic paint consists of pigment, which gives it color, and a synthetic resin binder, which holds the particles of pigment together in the buttery consistency we expect when we squeeze some paint from a tube.
Where it gets more complicated is in what exactly a manufacturer has in its binder and what else they might include, sometimes to cut costs. Surfactants are used to disperse pigments, and anti-foaming agents to stop the paint frothing as you use it. (Golden has a detailed article on foaming in acrylics.) Cheaper paints may contain fillers, opacifiers, or dyes, things that cost less than pigment.
Brands also contain different proportions of pigment, known as the pigment loading. If you've tried various brands, you'll have encountered ones with colors that are more intense than others.
See Also: Acrylic Problems: Separated Binder
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