It's not the pistils that you want to see turn brown, it's the heads of the small trichomes and you'd need a jeweler's loupe or magnifying glass for that. The pistils can turn brown long before the plant is ready to be harvested... And these ladies look like they've got a ways to go yet (4 weeks?) - I think you'll see them fattening up nicely. Just have patience - and use the time to buy yourself a jewelers loupe!
Good luck!