Depends on strain quite a bit, but also how many nutrients are left in the soil and if those nutrients are bioavailable or tied up. Too high of pH or too much salt can cause this earlier as well.

Chlorophyll production should not slow down until the plant starts to ripen. This occurs as the plant produces the hormone ethylene late in flower. You shouldn’t see leaves fade until the last few weeks of flower.

A lot of heavy leaning Indicas will turn purple early into flower and then on to yellow and red