I would hazard a quess that it is because you are thinning the acrylic with water. If you were to use an actual acrylic paint thinner available at the local hobby store or even "Michaels", you would have better results. These thinners are specially formulated to thin the acrylic, but not change the properties, such as water is most likely doing.
Also, if you were to paint all the areas that are to be colored with a base coat of white, you will be able to get a better color on the finished surface. As color is determined by the reflection of light through the paint, with a darker base coat you will end up with a darker finish.
Ken