If you're growing inside, the GDP might not turn purple. I grew it two or three times in a closet under lights and never saw a hint of purple, but after I started growing outside in the sun I had a bunch of purple flowers in late autumn-early winter. Not only the GDP but also Godfather Purps, Paris OG and Cheese all got intensely purple, but once the buds dry they look more or less the same color as any other dry flowers. At least, that's what mine did.
Edited to add: Also, I think you need at least a touch of cold weather to get the purple pigment to come out.