each real bloodline has different development charecteristics. So if you are buying tosai knowing how that bloodline and its shiro, size and conformation, sumi, and beni/hi develop is important. That is one reason so many great looking tosai do not turn out and some of the lame ducks do. Knowing the bloodline itself will help you determine that. Maurice has articles on his site about that issue.
http://www.koi-uk.co.uk/koiman/kohaku_hi_quality.htm http://www.koi-uk.co.uk/koiman/Kagezumi.htm http://www.koi-uk.co.uk/koiman/naokitosai.htm
very good for beginners to read thru and think about. That is why it is important to have a breeder's input about which koi truly have future potential, they hsould know how thier bloodline develops. I f they dont it is a sign someone else, like a parent or sibling developed the line and they have newly taken over, or just want to sell it. It will take some years for their eye to develop to know what they are looking at. A wrong guess also keeps them out of hot water with the buyer......and even with an experienced breeder's picks, there is a certain amount of cahnce and risk involved.
In tosai I do not really agree that you get what you see. In older koi, that is true. Maurice's site has a bunch or articles on subjects like this that make very good reading and education for buers who want to really learn about the subject. One great way to learn is to study the line, talk to folks who know how it tends to develop in terms of growth, beni, etc, then AFTER you have learned about that line in particular, do some tosai growouts.
maurice's site also has some articles about the different types of beni and how they develop and how to know if they are degrading or about to.