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Sure you could, but I think this is more suited for growers forums... What do you mean ratio of black seeds to white seeds? Are you talking about the actual physical color of the seeds? Because they should all be the similar coloring from the same strain. White seeds are probably immature anyways....
Starting seeds from 12/12 will make plants shorter. They will grow slow for a few weeks then start flowering. When it comes to seeds all you do is wait until you get a female and a male. Select the best male and collect his pollen. Pollinate a bud or two to make seeds. I suggest you to do some research and reading before you start. The first question you asked about the seeds makes no sense. Dark seeds are mature and white seeds are immature. If you do make seeds don't harvest them until they are dark.
Pollinate females at around week 3 of flowering or when a decent amount of pistols have shown up. From there it takes about 4-6 weeks for seeds to mature.