they change color as they get older some get more white than others and some get more orange,they usually lose there black color as they get older, as for growth it all depends on how u feed it and how clean the water is, if all is good u could expect anywhere bewtween half and a full inch a month until they reach about 6-7 inches then it will grow slower maybe an inch evrey 2-3 months
and the nuchal bump will only appear if its a male which you will see as it gets older, hope i helped

and gd luck