Pistils browning too soon

Pistils will also start turning if you touch them, make sure your not brushing by them when watering or inspecting

The first 2 photos look perfectly normal. I can see your concern on the 3rd photo though. I've had this happen before on one of my first ggg strain grows. Remember that not all seeds from the same strain will produce identical plants, in fact there can be quite a large variation. This is most likely a plant phenotype that needs more potassium (k) or less nitrogen than the other 2. An over feed of nitrogen or incorrect ph level can cause some nutrients to "lockout" and not be available to the plant. Maybe, new pistils will come through after a flush and then, after some extra pk/less N. I can't think of any other reason why the pistils brown so early it has only ever happened to me once.

I should have mentioned that when first starting to flower then you should cut the strength of your bloom nutes back then increase 100 ppm every week until you get back to normal feed.

Excessive nutients/overfeeding is usually why pistols brown early. Many say it's normal but it is not. No matter what they say. I have been growing many years indoors, many different strain and usually multiple runs from clone. If you keep your nutes in your soil at proper levels throughout your grow pistols will stay nice and white maybe a little hue of red/purple for the first 4 weeks of actual budding. 5th 6th week is when my grows usually start changing but by that time it is normal and buds are already huge. Cheers