My goal is always to try and get a red fleshed seedling but that is such a guessing game. I read somewhere once where the main apple guy for developing new apples (like HoneyCrisp) at the Univ of Minn who I think is named David said they use 1 out of a 1,000 seeds and then reduce that to 1 out of 1,000 of those before they put a trial number on them and then a name. After I posted that someone told me it was not true so I do not how many seedlings big places try. I just know for me I will put 50 to 200 seeds out and see what happens.
As soon as they sprout if the leaves stay small I pull them out because odds are it will be crab apple size fruit. If I see reddish tinged leaves or or extra large leaves or out of the ordinary growth then they are kept around
If you have grown apples from seed you know it can take forever to set fruit the first time, maybe 7 years on average. I try to speed that up a little on what I think might be promising by grafting them onto B9 which will force fruit a little sooner so I can see if it is worth keeping around. Nothing in my orchard is on B9 thought because we have way too many deer, I grow my production trees on M111 or M7 or G30.
The photo I posted is probably the darkest red fleshed seedling I have grown and is still known as RF2009#3 which tells me that it sprouted in 2009 and when I look in my notes it was the third seed I planted from a Rubaiyat x Golden Russet cross. I have a couple of friends who have given me some great suggestions on a name for it though.
The one I grew from the Pink Pearl x Golden Russet cross and named is the that people rave over and we sell out of it. It is sweeter by far than #3 above but #3 is so juicy that when you cut it the juice flows like a blood orange. This year we are going to make hard cider out of #3 just to see how it is.