Just wanted to see if anyone can shed any light on this?

To be honest, you are very patient. To be blunt - pull it out and plant some new seeds. Also looks very wet, try not to have standing water in the saucers under your pots. If the water in the saucers isn't absorbed/sucked up within 30 minutes, at most, you will need to empty the saucers, otherwise you may get fungal problems from extended moist conditions in the soil. Also, another tip - next time fill your pots right up. This will provide much more room for root growth and overall plant size. Hope this helps..... Organoman.

I had a White Widow autoflower seedling where entire plant turned that color. First signs of it started around week 1-2 by week 2-3 the whole plant was that color. At that point, the yellowish color started to retreat with the green starting back at the leaf stems and making its way toward the tip. All new growth came in normal. It grew very little if any for about a week, but now coming to the end of week five it is now one of my healthiest plants. So what was it...? Beats the hell out of me! It was being started in Fox Farms' Coco Loco which is reputed to run a bit hot, so I'd blame that except for the fact I had three other seedlings of the same strain, from the same buy, all planted on the same day in the same soil and none of them did anything close to that. I ended up just chalking it up to some weird genetic thing and decided I was good with just being happy it self corrected :) I'm sure you've already researched any number of possible causes for this, but if none of them seem to apply I'd just be patient and keep my fingers crossed. Good luck with your grow! PS: I'm no expert (just five weeks into my first grow) and I know pictures can be deceiving, but it looks like your soil might be a bit too moist. On the other hand, I am probably the worlds most paranoid grower when it comes to worrying about overwatering so you need to take anything I say in that regard with a grain of salt. When they're that size I am literally watering with measured doses from a syringe!

That small at 6 weeks. Something happened in the soil that probably stunted and or killed off growth. Check your runoff ph. 🤘😤