Acrylic paints are water-soluble when wet but dry to be waterproof and matte. The binder in acrylic paints, acrylic polymer, is white when wet but dries transparent, which is why acrylics tend to darken slightly as they dry.
We recommend using a painting medium specifically designed for acrylics, although water can also be used for thinning. With mediums, you can easily adjust the thickness, gloss level, and drying time of the paint, and the color remains stable without breaking down, especially when using large amounts of water. Varnish can alter the gloss level of the finished painting.