Why are my jalapeno seedlings turning yellow?

Question

I planted these jalapenos about a month ago. I am growing them indoors before I transplant them to my outdoor garden. They've been doing fine until the last day or two, when the leaves have started to turn yellow and even brown. I water them about once a day, maybe a little less often. They are planted in Miracle Gro Seed Starting Planting Mix. I have taken them outside a few times to help them get used to the outside.

Extra info that seems relevant based on the responses I'm getting: I live at an elevation of about 200 feet in the Northern hemisphere, 40 degrees north.

See the pictures below. Please note these are two different plants:

Answer 1

Looks like sunburn/cold damage, as suggested by others. I don't know where you are in the world, so its hard to be definitive, but those seedlings look a little small to be thrust into the outdoors in cool spring temperatures but high uv levels (northern hemisphere) - best to wait till they've formed proper small plants (with a good half a dozen sets of leaves) before hardening them off for outdoors - unless you live somewhere very warm. When you do start hardening them off, start with them in shade initially, not allowing sun exposure between 11 and 3 pm as a minimum.

Answer 2

I looks like sunburn to me, but good news, those damaged leaves are just cotyledons, and they were going to fall off naturally any way... I have gotten this from peppers that I have started in domes in the sun, or in peppers that I have started inside and moved them out...

In my experience it just isn't even possible to harden off tomatoes / pepper seedlings with no damage (I live in higher altitude with lots of sun, so your mileage will vary)... but you can decrease the amount of injury by starting to give them a little morning sun... then a little more, etc... and if you a little patient then they will be fine, if you are very patient they will be great.