Some of my bush beans are light green, is this a problem?

Question

In the picture you can see five Romano bush bean plants in my raised bed. These are all exactly the same variety; in fact, they came out of the same six-pack of seedlings from the nursery. Yet, four of them have bright green leaves, one has dark green leaves, and one has some of both.

What's going on here?

All of the plants appear healthy. All of them are currently starting to fruit. They get the same sun, give or take a little during the "dappling" part of the morning. None of the leaves are turning yellow.

Possibilities I can think of:

• Chlorosis, and I need to add iron to the soil
• Needs fertilizer, particularly nitrogen
• Overwatering (they're on the same soaker hose, but it could be delivering more/less water to specific spots)
• Underwatering (same)
• Pests attacking the roots
• something else
• It's not a problem, ignore it, bean plants are like that.

So, thoughts?

Answer 1

Since I never got a response to this, I had to figure it out on my own.

The answer turned out to be Chlorosis. The bean plants weren't getting enough iron, because the pH of the soil was too high (alkaline). After several rounds of soil testing, adding a bunch of chelated iron to the soil, and several acidifiers, I brought them back to health.