Dont harvest by pistil color alone.
In the 70's they used to harvest when the pistils were like 80% orange/brown, but that means that alot of the bud is still not at its peak.
Invest in a jewellers loupe or a trichome scope... when the pistils start chaniging color, use the loupe/scope to check out what color the trichomes(crystals) are..
Each strain has its peak potency, and can give different effects depending on when it is harvested...
for example 80% cloudy trichs with 20% clear trichs would give you a clear 'up' high.
Whilst
a trich ratio of 70% cloudy + 30% amber will give you an 'up' ish high with a rather stoney effect.
experiment with your harvesting, make notes of the trich ratios each time you pick and see which high you like the most.
I tend to pick at 70% cloudy 30% amber, as i like the stoney feeling and the CBD and CBN from the amber trichs helps my pain relief.
for your original question, its natural for some pistils to change color before others, bearing in mind those are
prolly the first pistils that showed themselves, therefore theyre older than the other nice clean white hairs, and evidently mature before the newer growth.
Some pistils will also change color during growth for numerous reasons... ie brushing past the pistils can make them change color as they are damaged, as can foliar feeding esp if you do it when the lights are on and it scorches the pistils hairs somewhat.