Here is something I wrote before on the Sussex breed:
The Sussex is a British breed that was named after the English county from which it originated, Sussex. This dual-purpose breed is known for their sweet temperament, often regarded as one of the best chickens to have for companionship...
The white will go away over time, and usually its the cockerels who have more white on their chest when they are young and it takes them longer to grow out of it than the pullets.