Koi Color
The Yellow/orange coloration on angelfish is poorly understood, so we will attempt to make this a bit more clear here.
Genetics
The genes for gold and gold marble are primarily responsible for these colors in angelfish. If you see yellow, orange, or a shade of these, then the fish likely contains one or both of these. Marble also appears to have the ability to add some color, but it there is any great amount, then gold or gold marble is likely present too. Genetics determines the potential for color, but not the actual amount you will see on a particular fish.
Epigenetics - From decades of selective breeding with lots of orange fish, we have come to the belief that epigenetics is the primary driver of the amount of color potential on the fish. Genes can be left dormant or they can express depending on a 'epigenetic marker' that is placed on a gene. This marker is influenced by environment, and the amount of color the parent fish had. This is why breeding the best to the best is so important. You can't breed for more gold genes, but you can increase the number of these markers and control how easily they express. If you don't carefully select for these traits, then each generation will progressively get less color. Epigenetics is the reason why there is so much variation in the amount of color on fish containing gold and/or gold marble genes.
Environment
You control the epigenetic expression through the environment you provide. Remember, you turn the expression of these genes on or off, depending on the stress levels of the fish. Poor environment = poor color, good environment = good color, great environment = great color! We will try to list these in order of importance, but much of this is just a guess. If many aspects of the environment are a little off, the accumulated stress will be enough to suppress color to a great degree.
Water: This is very important. If you have bad water out of the tap, you will struggle with color. We can't pinpoint the important aspects of the best water, but we do know that different water will produce different levels of color. Sometimes the only way to get good color is to make your own water. By using a reverse osmosis filter and reconstituting the water, you can manufacture whatever conditions you want. Unfortunately, there is no formula to follow. It will take a lot of experimentation.
Food: This is important in a few ways. First, it has to contain carotenoids, because these are the compounds that the fish store in their skin that express as yellow/orange. They cannot produce these pigments, they must eat them. When people say they don't "color feed" their fish, they actually do. There are over 400 different carotenoids and almost all foods contain them. Without them, they could never have a fish that expressed even a tiny bit of these colors.
Second, feeding amounts have to be precise. Too much, causes stress and reduces color. Too little causes stress and reduces color. We pay more attention to feeding amounts than almost any other aspects of husbandry. Careful observation is important. Fish should eat vigorously and should never be fed to the point where they will not be able to eat more.
Lastly, food should be fresh and the closer it is to a real animal, the better it is. Highly processed commercial foods are great for convenience, but don't expect the best results if you feed these exclusively. You can increase the carotenoid levels with these foods, but by themselves, they will be inadequate.
Stripeless Gene: Angelfish have a few skin layers and the carotenoid layer is beneath other pigment layers. For this reason, the stripeless gene is important in being able to see the stored pigment. Homozygous stripeless makes the top layer translucent, so you can see underlying pigments. The most clear and bright pigments are seen in the most translucent fish. If you want the best fish, you have to pay attention to many things when selecting breeders.
Fish Husbandry
Fish are always nicer when you change more water, keep fewer fish per gallon of water, feed with great care and make sure the fish are unstressed. Stress can come from hundreds of things, and if you don't minimize these, your fish will not be as colorful as they could be. Sometimes, it can be very difficult to recognize stress and even harder to know what caused it. That is the challenge. Color is one of the biggest indicators of stress, so when a person says their fish are unstressed, but have little color, chances are good they simply don't recognize the stress.
A few of the many things that can cause color to fade - some of them do it very rapidly: Netting, shipping, aggressive fish, overstocked tank, lack of water changes, temps too high, light too bright, tank too sparse, pH too high, water too hard, poor quality food, overfeeding, underfeeding, feeling insecure, poor filtration, or lack of established biofiltration. They are many others, but you should get the point.
Age
Young fish have less color and the color is more susceptible to change. As they age, the color becomes more fixed and less likely to improve or degrade. However, with enough stress it can still go downhill.
Buying Fish with Color
If you buy fish without color and they are very young (less than 2 months old), you may be able to increase it quite a bit, but never as much as if you received fish that already have a lot of color. If you get young fish with great color, you can easily stress them enough to cause fading. The safest bet is to get older fish that already have great color, making it a bit less likely that they will fade as much as young fish can. Remember, receiving great colored fish, does not mean they'll stay that way. The immune system uses carotenoids as anti-oxidants when the fish is under stress. You don't want this to happen. You want them to store the carotenoids, not use them to combat stress.