is a common finding in newborns and phototherapy
is the correct mode of management. However your suspicion of you child developing a dark skin tone due to phototherapy is not possible. Usually, when infants with yellow skin tinge due neonatal jaundice are subjected to phototherapy, there will be a change of skin tone from yellow colour to bleach like colour due to change of accumulated bilirubin
into lesser metabolites and this is entirely normal. But phototherapy as such does not make a child look darker.
The myth that you have mentioned may not be true as there are no accurate studies to back up such claims.