The Blue is a cockerel. The Mottled is a pullet. Just look at the difference in the combs. Pullets don't start to really develop in the comb until they are getting close to laying eggs.
A rooster at 3 months old will have a bright pink, three row comb. A girl will have a slightly raised single row of "peas" and still be yellow or pale pink.
Boy comb
Girl comb