I really think that the comb itself doesn't get a bright red until they are finally ready to lay.
The comb can stay pretty dark until they start "the change"
Also, I really don't think that you should start looking for RIR eggs at 15 weeks. If you have hatchery birds, I would think you should start looking no earlier than 19 weeks of age. If they are fancier breeder birds (so, better personality, more years of laying, and simply very pretty) they might not start laying until 6 months old, or even much older.