Black water; Very dark or black water is caused by excess rotting vegetation on the pond bottom. When oxygenating plants have not been trimmed back enough or yellow and brown waterlilly leaves have not been removed, they build up and decay on the bottom of the pond. Remove the decayed matter and the pond will clear up in a few days to a week.
Oily patches on the surface of the water are caused by waterlilly leaves dying and rotting on the water surface. Remove the dead leaves and lay newspaper on the water surface to remove the oil. Some fish foods will leave an oily residue on the water surface. This is not detrimental.
Foam on the water is an indication of excessive amounts of organic material in the water (dead plant life, feces, fish food, etc). Left untreated it decays and encourages bacteria to grow in the water that will kill your fish when it is weakened or hurt. Treatment is simple. Remove 25% of the water in the pond and replace it with fresh water (don't forget to dechlorinate the water) each day for 3 days.
The color of the water is a very good indicator of its quality. It is not the only method to use however. Checking the pH, ammonia, and nitrates will help you to prevent your fish from suffering illnesses. Here are some visual signs that will help in determining water quality:
Green water is associated with tiny algae feeding on mineral salts in the pond. You can use barley straw near an oxygen source such as a waterfall or where water enters and oscillates the flow of water. As the barley decomposes it prevents the algae from propagating. You can also purchase UV filters/clarifiers if the problem persists. This will not effect string algae however. Depriving the algae of sunshine by covering 70% of the pond surface with plants will also eliminate the problem.
Brown water is caused by silts and sediments being disturbed in pots and the pond bottom by fish or other agitator. It can be remedied in the short run by flocculators (clarifiers) which make the silt clump together and sink to the bottom of the pond. Covering the dirt in a pot with a layer of 2 inch diameter river rocks will keep the fish from disturbing the material in the pot. Koi are problematic in that they constantly look for roots and bugs and stir up trouble. Brown water can also be caused by tannic acid - which is released into the water by pine needles, oak and maple leaves. Promptly remove these or a build up of tannic acid will kill your fish.