Any suggestion for beginners

Some of that may be temp/genneics erlatd (the purple or red hues). If temps hit 66-68F and below, you get fall colors in plants.. some genetics display it regardless, too. But, definitely have some nutrient issues going on too. some interveinal chlorosis down below? can't really see.. looks a bit pale, but that also could be an illusion of grow lights + photography. need more info / better pics of damage.

Had exactly the same symptoms on this plant after overnuting

likely overfed and to much light need more downtime. 20/4 is a bit much. I like to keep it max at 18/6 for autos. Should try and calculate your DLI your giving them at 20/4 Don't want anything over 35-40 for most plants and environments. Some can take more but you need to add in extra co2 and have other aspects dialed in. I don't think by the looks of it your hitting that 40 mark given the stretch of your plants but still should try and check it to know. Good Luck!

It looks like you are either experiencing some issue with a build up of nutrients in your media and/or too much light. Light is an easy fix. You should be around roughly 800-900 PPFD at the top. You can download a PPFD reader on your phone to find that number. As for too much salts being built up in the media, I suggest you give a nice flush using straight pH balanced water. Do this at a rate of two gallons per every gallon of your pot. Apply the water over about 10 min to allow for it to leach all the way through your media instead of a single dump which can follow the path of least resistance and not properly flush your media. On the next feeding, follow your nutrient manufactures recommendation and you should be good to go. Make sure your properly pH balancing and utilizing proper watering. Water enough to have about 10-20% run off out the bottom every time. This will eliminate the build up of salts in your media. I hope this helps.

the yellow leaf can be quite normal in this phase but yours seem to be a little crispy.. Maybe sunburn or salt buildup. My advice is to start flush now, for about a week

Take some runoff readings. You're at a stage where salt build-up can be common. You may need to flush.