Attached my chili pepper plant's leaves. Instead of dark green, it changes yellowish. What's the reason for it please?

Background, these are Thai chili pepper plants I started from seeds last year in containers. I just moved them in ground since I noticed the fruits are very tiny lately after fruiting normal size in fall and winter. Someone suggested to move it in ground so I did

Yeah, leaf colours fluctuate.
Above looks like an older leaf.

How 'bout a picture of entire plant?
Above sample is too small to tell much.

Personally I wouldn't be concerned yet.
Is there any new growth?

What was the process in setting these out?
What's the garden soil like?
How prepared?

Maybe a lack of iron?

that leaf lacks iron but we need to know what the other leaves look like. If its an old leaf, the plant is merely harvesting nutrients before dumping it. If its a new leaf, the plant may need fertilizer. Or if the plant was fertilized, it was too much. Or the plant is being overwatered. We need to see the rest of the plant.

Here's the whole plant. It's about 3 feet tall. I read on the internet and it said it might be under water so I just watered it deep today. I just added compost to the ground last year. I'll add more in a few months.

All my pepper plants are not fruiting now. Someone mentioned that they take a break since they've been fruiting in fall and winter

Personally, I think I would attempt to divide that clump some.
I had some clumps like that... After setting out and watering enough to moisten rootball, I dug out smaller clumps, like 3 or 4 plants... and set those out... Later, I may divide further.

My plants are a little unhealthy looking too... I never expect them to instantly improve... time takes time... Being out of the pot will help... But not over-night.

I'm seeing flowers on mine... You probably will be seeing flower buds at any time.
... And... those older leaves? I don't see a problem.
The newest leaves look fine.
Now it's just waiting for the plants to continue putting out fresh growth.

Dividing isn't absolutely necessary but pepper plants do okay being ripped apart and replanted. You can even plant them deep to get rid of excess stem although, unlike tomatoes, they won't root on the stem. If you leave it as is, it will require more water and nutrients because there's a lot of competition going on there.

Water deeply but in a bigger area to encourage the peppers to roots outwards away from the plant to lessen competition and add some fertilizer.

I finished ripped the plants apart and replanted them in ground so each has its own water drip. I see one plant started having fruits but the fruits are still small but red already. Hopefully the fruits will be normal size again.