| The sunlight issue is only an issue if there is NO natural light. Koi must make certain colors in their skin ( red/orange and yellow) from the food they eat. They proccess this in specialized cells within the skin. If no light is present, the chemical reaction can not take place over time. Typically this is due to lack of 'ingredients' like the algaes and also vitamin issues ( vita B , D). And the yellow simple shrinks to expose white skin.
Too much light has the same effect as other reactions take place in the skin, eventually washing out luster and color cells. This is why a partly shaded pond is ideal and why high quality fish should never be exposed to full sun in very clear water all day long. It will ruin the color over time unless the water has some degree of turbidity or the water is very deep.
There is a common myth heard here in the USA. And that is that the Japanese 'prefer ' kohaku with a light persimmon orange and 'prefer' their yamabuki to be of a more light yellow as opposed to a strong yellow. This is an example of both language barrier to communication and an in ability to absorb information based on the listener's current level of understanding.
The next level of understanding about these statements is that the colors described above are early stages to LOOK FOR when buying a younger tategoi. That is partly true, but misses the 'forest through the trees'. It is something to look for ( lighter yellow in young yamabuki) and green and cream casts are hints, but the advanced koi kichi is looking for the right skin and not the color shade itself. The Jitai must be right. This means the type of skin and the color within, that, when present together, give the adult look you want.
Yamabuki breeding is very efficient and profitable because 80% of the spawn is very sellable. If you compare that to sanke, where the sellable percentages are tiny, yamabuki represent a much better investment and effort for a breeder as a cash crop.
But the 'failure rate' in yamabuki, as they age, is very high! Higher than sanke! I suspect that the same low percentage of true adults as any other variety. This might not sound right as you may own a butter yellow yamabuki with average skin that HOLDS its uniform color well. But the point is, it does not improve, it just manages to hold it's youth as luster declines. Not a bad thing but not the upper 2% of development. That upper 2% will have -
* uniform and refined color in three dimensional filling.
* tremendous luster covering the entire body ( this is THE most important element)
* Fukurin skin in the skin surrounding the even, same size scales.
Hopefully the reader can see that this is a much more evolved animal than the butter yellow pond fish?
In failing, most yamabuki will either become too yellow and uneven in color with orange spots on the head and body. Or they will wash out with white showing at the fins, head, face folds and along the spin. And many will remain yellow but loose all the luster that is the key element for Hakari muji. The last description is very common and often hobbyists are very disappointed that there very yellow pet fish did not win in a koi show. Unfortunately no fish without metallic sheen can win in the hikari muji class.
DO keep in mind that a yamabuki is a ONE colored fish. This means NO white or platinum at the fish tips, mouth or head/skull. Sometimes this is just poor color cell coverage but often the fish is actually a hariwake 'wantabe'. Many ymambuki have been crossed with platinum purachina to get hariwake. So in some percentage of offspring, as they age past sexual maturity genes start to express themselves in other ways. Both in the skin itself AND the color cells and pattern. We call this finishing in patterned fish. But after finishing what we see as 'decline' is actually new gene expression.
Yamabuki is a simple, yet it can be quite sophisticated. - JR | 
05-14-2008
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