One of the stumbling blocks for strangers to the acrylic realm is the slight colour shift that occurs when the paint goes from wet to dry. The degree of change depends largely on the amount of pigment in the acrylic itself.
When wet, the acrylic emulsion has a white appearance because of the water it contains. The way that light travels through the water causes it to appear opaque. As the water evaporates, the paint film clarifies, leaving the pigment to be fully developed.
Artist-quality paints contain a significantly larger quantity of pigment than the student or economy varieties. The higher the pigment load, the smaller the colour shifts between the wet and dry stage. Lower quality paints also tend to contain more water and fillers, opacifying the wet paint further.
Matting agents and fillers will also cause the wet paint to have a more bleached-out cast, though this will also decrease significantly when the paint is dry. If you have concerns about matching a tone exactly, the best way to work is with a test area and a blow-dryer.