Many times hobbyists looking to buy small koi will select koi that appear to be more finished. And by finished I mean a deep red color, all the black is in and the white is bright. While the finished koi does have immediate appeal, often times in young koi the finished look does not last . As the koi gets older the color can break apart and fade. You then end up with a koi that is not nearly as nice looking as when you bought it. You can avoid this by purchasing young koi that are not yet finished. It is not easy tell the future of a koi and you need to develop an eye for what the colors will do. Developing this eye usually takes years. You need to watch lots of koi grow and develop. Selecting tosai with a lot of potential is a skill that breeders can often take a lifetime to perfect. But there are a few basic things to look for that can help you pick koi with more potential.
First look at the Hi (red). On young koi this is often a lighter more orange color and it will turn a darker red as it gets older. The lighter orange is often referred to as "soft". A good example of soft red can be seen in the first photo of the koi above. Young koi with a dark red Hi are usually male and have a higher chance of losing their Hi in a couple years.
Next look at the Sumi (black). Sumi is much harder to predict. It can come in, fade away, and then come back years later. Sometimes it takes years to come in, sometimes it never comes in. Usually if you have hard water (a high gH) then Sumi will develop faster. On young koi underlying sumi has a light bluish color. This is developing black that has the potential to come in later. A young koi with lots of dark inky black is again probably male and stands a better chance of losing the color sooner rather then later. The middle photo above is a very good example of developing Sumi.
Lastly we look at the white. On a younger fish, the white can be a bit pinkish. This is perfectly acceptable as the color plates are not fully developed. The young koi can have very blurred sashi (leading scale edge of the Hi). This is caused by the thin white scale covering the red scale under it. As the koi ages and the white thickens the sashi should tighten up.
These are just a few basic traits to look for when shopping for young koi. Of course there are truly no guarantees when purchasing young fish, but even when they don't turn out as expected you learn a lot by watching them develop. And over the years, as your eye develops, your selections will improve.