Flushing a plant a few weeks before harvesting is done to mitigate the salt buildup inside your plant. Experienced smokers can taste that if you're using mineralic fertilizers. Especially too much nitrogen (dark green leaves) will make the smoke harsh and chlorophyllic-tasty.
You don't necessarily have to do a real flush (=pouring 3 times the vessels capacity in water through the pot and throwing all excess water away).
You could as well just feed it plain water. After some time all the leaves should be back at a normal green colour, bottom fanleaves will wilt and finally all fanleaves or greater leaves will start yellowing, stems will become weak and many colas will just fall over, the whole plant looking somewhat lifeless.
BTW as you see from the pic if a plant drains its own salts the leaves will start yellowing homogenously.
This is totally different from the yellow spots that did appear mostly around the veins of your dark coloured fan leaves of the one automatic.