Although it is true that
acrylic paint won't exhibit
colour change over its lifetime, this is only true after it has dried. Unlike
watercolour or
gouache, which will lighten as they dry, acrylics will dry slightly darker than their original colour when wet, so this should be taken into account when mixing your colours. The reason for this is because the binder in acrylic paints is white when wet, making it appear lighter, but dries clear. It is also a possibility that the paint can become less
plastic and crack if they become cold enough, but unless you keep your paintings outdoors this shouldn't be a problem. There are essentially two methods for using acrylics: the watercolour technique and the oil technique.
Watercolour Technique
Since they are water-soluble, acrylics can be thinned with water to achieve watercolour effects. The paint will become more transparent as you dilute it, but by using too much water it is possible for the pigment and binder to separate, giving it a grainy appearance. If you need to add more than a small amount of water to achieve a viscous consistency, such as for washes, it is advisable to use a medium specifically made to do that. Another benefit of using a medium is that the strength of the colour will be retained. A soft watercolour brush can be used with this technique, unlike with the oil technique, which would benefit more from a stiffer brush. One distinct difference acrylics has over watercolour is that it can be overpainted without disturbing previous washes, since it loses its water solubility once dry. When overpainting with watercolour, the wet brush will cause previous dry layers to become wet and bleed together. Therefore, using acrylics in this manner can be a benefit, or a hindrance, depending on your preferences.
Oil Technique
Acrylic paint doesn't have to be thinned and can be used directly from the tube. This gives it a consistency very much like oil paints and allows for lots of texture. The paint will retain brush strokes very well, particularly if a stiff bristle is used, and palette knives can be used for impasto work. Many painters prefer to use brushes with longer handles when using this method because it allows you to stand further back while painting and visualise the piece as a whole. Acrylics have many advantages to oil paints including: easier clean up, the canvas or board does not require a coat of size, and low drying time. Mediums can also be added to the paint to add gloss, extend the paint, add special textures or increase the drying time.
When using acrylics it is very important to clean your brushes properly because it can be quite difficult if the paint dries since it will literally turn to plastic. This does not mean you should let them sit in water too long as it will damage your brushes. Soap and water will clean them nicely without the use of any solvents like with oils. Rub bristles against your palm until the water is clear.