Help with young plant

This is an Asian Long Bean. I don't understand the sudden yellowing. I don't think beans need nitrogen added and this is a Promix soil.  Any ideas on how to treat/prevent?

I am also new at starting seeds in my greenhouse this year and noticed all my lovely seedlings getting yellow, so I did some research and found out as soon as the seedlings grow two true leaves.  The first two are Cotyledons, or seed leaves and the second set that actually have the look of that plants kind of leaf are the true leaves.  I did not know any of this, so lots of my seedlings look kind of sad now.  Even beans can have a  nitrogen deficiency.  You want to give 1/2 strength fertilizer to your seedlings.  My understanding is the new leaves should grow nice and green, but the yellow leaves will stay that way.  I started flowers in doors before, and had no trouble what so ever, but veggies are proving to be a bit harder.  To much light, to hot, water enough to much, replanting to avoid over crowding, hardening the little guys off, it's definitely a learning process.  
Beans are relativity easy to grow, so unless you have a very short growing season, you can always direct sow them in the garden.  I have way more success with direct sowing.  I planted tons of peas last fall and they are producing like crazy, and I wanted to give them as much time as I could before planting the summer crop, so I started lots of seeds in my new greenhouse.  If my seedlings don't pop back, I will just throw some seeds in the ground.  I was looking forward to sharing my seedlings, so I hope they get healthier.  I have been using organic fish and kelp fertilizer,  with some worm castings at about 1/2 strength.  I don't know if this will do the job because it's to early to tell.  Good luck to you, I hope your beans do well.

I've noticed several of my transplants have begun to lag in growth & vigor, which I assume is due to them using all of the available nutrition in the small containers. Some, like the borage, looked similar to your plant. A couple of days ago I decided to dilute some comfrey/weed/urine "tea" to water them with.

It hasn't been long enough to tell if I helped or hurt them, but I hope it will give them a little boost to hold on until I can get them in the ground. I tried to keep it at a 1:6-8 ratio, but it was a little hard to monitor with dipping a cup in the stinky tea for the hot pink, plastic watering can (guys can like pink, too, LOL), then filling the rest from the buckets of rain water.

I would add, be patient with the long beans. They take a LOT more time to develop and grow (and to set flowers/fruit) than normal pole beans. Like to the point that you may find yourself googling things like "why are my beans not setting flowers" in desperation. I planted bushes, pole, and long beans all at once, the bush beans are a far-off memory, and the pole beans were done and ready to pull out by the time the long beans started climbing. I wouldn't go nuts with nitrogen, if you're using a mix you know you can trust, but I would make sure they're getting as much light as you can give them.
The one pic almost looks like they got sunburned, did they go from shade to full sun immediately?

Tereza Okava wrote: did they go from shade to full sun immediately?

They went from a warm grow room under fluorescent lights to outside.  The temperature of concern was more of a low temperature at night.  

I'd also say sunburn.

I will put these under a shade cloth.

I agree with above posts, plants seem like they did not harden outside.

They are in a bit of shock.  

Stabilize night temps as best you can.  Look up hardening plants and you will find countless information from the simple to the luxury.

While beans grow best in low nitrogen soils it is because they form a relationship with bacteria that make viable the nitrogen from the air and supply that nitrogen to the plant and the plant gives back sugars.  This takes time so as stated a bit of low nitrogen fertilizer can be necessary early on but careful.