Discoloration of the walls of a swimming pool is due to incorrect water balance.
If the walls are slimy to touch then you have algae. This will need to be killed with a large dose of chlorine. See Algae help for more information.
The other alternative is a stain. Stains are often caused by various minerals or metals falling out of the water due to an incorrect water balance. The first step is to check your water balance. In most cases the pH is the problem. Black copper stains or brown manganese or iron stains both predominantly occur when the pH is high. Reducing the pH in most cases will remove the stain back into solution. If you have problems with re-occurring black stains, get your copper levels tested and do not use algaecides as they contain high levels of copper. This is common in many ioniser based systems as they use Copper/Silver electrodes. This is one reason why ionisers are not sold in every pool shop.
Salt chlorinators, due to the sudden change in pH they create, can often produce brown iron/manganese stains, which will start opposite the return jet. Again, adjusting your pH will solve this problem.
Once the water balance is correct any remaining stains can be corrected using a stain remover or metal remover.
Staining is very common in vinyl and fibreglass pools. Brown dusty stains on the liner are best removed with an organic acid stain remover.