Find a copy of George Henderson's "Farmer's Progress" as a download from the internet. It is an old book, but read the section on large scale breeding of you own replacements. It can be done, but the advantages of buying pullets have already been outlined to you.
If you go for breeds that lay "coloured" eggs (and brown eggs are referred to as coloured by some, so be careful) your production levels will be much lower than commercial strains of hybrids, consequently your prices will have to be higher. That then gives you a limited market compared with your same, probably better, quality brown eggs. I would steer clear of white shells, most non supermarket buyers do not want them.