The ability to turn purple is genetic. You can make it MORE purple by dropping the temps about 2 weeks before harvest down into the low 60s at night. You can also treat it with SnowStorm, which makes purple pigmentation come out more clearly, and also give a more crystally look.
Don't worry about the purple yet. At this point, concentrate on plant health. Improving the appearance of your buds is something we will worry about when you are ready to flower.