Polymer or acrylic is the medium/vehicle/binder for acrylic paint. It is white when wet, but dries totally clear and glossy. There are no white additives in the binder, but the white appearance is due to a microscopic bubbling that disappears when dry. So when you paint with acrylic it is lighter when wet, due to this whitish phenomenon. Then when the paint is dry, this binder turns clear and the color turns into its true hue. Here is an interesting comparison from music that helps me when I paint. Musical scales consist of the same note patterns, but they change in octaves. So just like I would transpose one octave into another, I paint about 10-15% lighter then what I want it to look like when dry. In other words, I go up one octave in value when I paint. The more gels or mediums you add to your paint color the greater the difference between it’s hue when wet as opposed to dry.
If this is bothersome to you here are some other options. If you don’t mind working with the paint a bit thick, then try adding at least 50% Golden’s Light Molding Paste to your paints. The paste is white when wet, and stays white when dry, so there is no change (or at least very slight) in color between wet and dry.
Another option if you don’t like the hue change, and don’t want to work with pastes, is to use the acrylic in washes like watercolor. This technique is best accomplished using an absorbent surface such as watercolor paper, or some of the unusual acrylic grounds that are available (like Pumice Gel, Light Molding Paste, and Absorbent Ground). Add at least 50% water to your paint. After painting with these diluted washes, the color stays pretty much the same hue when dry. This is because most of the acrylic binder has been diminished with the addition of water.
Pictured here is one of my paintings that uses all three techniques (1) acrylic painted lighter while wet, (2) washes on absorbent surfaces and (3) adding Light Molding Paste to the paint)