After the surface of the paint has begun to congeal, the areas which are dry to the touch are permanent and can no longer be dissolved. Clean up tools and palette with soap and water. Always use synthetic bristle brushes with acrylics. Nylon brushes work well. Natural bristles will draw in the moisture which causes them to swell, then break down.
Color is something that can be frustrating to deal with in acrylic painting.
The polymer binder in acrylic paint has a semi-opaque milky look before it is dry. As the paint dries the paint becomes transparent. This taints the way one sees the color. When in the liquid state, colors in acrylic paint appear to be lighter than they will be when dry. One must account for this discrepancy by visualizing how the color will appear in advance.