At six weeks you are looking for a patterned koi, by then the beni will be in it's place, but looking very pale, orange/brown. white skin looks grey, as mix of black and white.
Normal practice is to cull out all yellow fry at 3 days, then if you have a good growing pond cull for kohaku pattern at 4 weeks, for me I have to wait 6 weeks.
Then on subsequent culls for quality.
If you keep the yellow fry, most will look to be a mix type sanke/showa, but a few will turn into good modern showa.
It just depends on your space and what you want from your breeding, true type koi, or just something to sell.
I've not got any good shots, but this gives the general idea.
Maurice.