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The appeal of the color shot is those blue eyes... He's the next Steve McQueen
Green is an interesting color both psychologically and physiologically. The reason it can be so distracting stem from the fact the rod cells that cover 95% of the retina are monochromatic and sensitive to a narrow band of greenish light. They are also 3000 times more sensitive than the color sensing cone cells in the center of the visual field. So while we are focusing attention on those blue eyes and red lips with the center of our attention those rod cells are screaming at our brains, "HEY COME LOOK AT ME I'M BRIGHTER."
Color balance in outdoor shots gets skewed when a subject is sitting under trees or on grass because of the green light bouncing off the foliage. Using custom WB off a gray card near the face helps somewhat but can't fix the root cause of the problem of color balance which is the mix of Daylight balance sun, cooler skylight, and greenish shadows due to the bias in the bounced fill. A B&W conversion is one option but it's like putting a bandage on the foot after shooting it. The better solution is don't shoot yourself in the foot in the first place: if you can avoid it do not shoot under trees, near trees, sitting on grass or any situation where the foliage is going to bounce greenish light onto the faces.
The problem when shooting is that you will not see the green bias because your eyes adapt to it. But the camera set to Daylight WB will and record the face with a odd flat gray look and oversaturate the green foliage — the problem I'm seeing in your color shot. A way to be aware of color casts is this trick I use:
1) Set Camera to Daylight WB
2) Shoot a Gray card filling frame
3) Set Custom WB off the card
4) Shoot the card again
5) Compare the two frames
What you'll see in the playback is this:
Color perception is a relative thing because the brain adapts. You'll not see the green bias in the Daylight WB shot in the playback until you compare it to the Custom WB shot and your brain intellectually accepts that second Custom WB as the "Correct" color. In addition to being able to realize there is a color cast in the light you will have with those steps neutralized it at capture.
Thinking about adding flash to correct the problem? That will make matters worse. When setting Custom WB what happens is the camera skews its normal Daylight WB towards magenta. That means when you add flash relative to the new camera WB it looks magenta and a bit blue because its color temp is higher than daylight.
When you open the file on the computer the highlights lit with the flash will look too magenta, so what will you do? Add green to compensate, which will shift the shadows, which were neutral, towards green. It's like a dog chasing it's tail. The only way to fix it is via two separate layer and masks. Again it's a bandage over the bullet wound in the foot. Better to learn how to handle the gun more skillfully.
That said, here's how to bandage the wound:
Step one is tone down the green, which I did by adjusting with Hue / Saturation. I simply selected yellows and cut the saturation which changes the hue to where the rods of the eye don't react as much to it and its not so annoying.
Beyond that the problem with the shot is that the dark background isn't an ideal choice in terms of tonal dynamic for the white clothing or the lighting pattern on the face. When you put a white shirt on a dark background it will distract attention from the face because it contrasts more. If you want the face to be the star of the show when the subject is wearing white you want to: 1) Put them against light background so the white shirt blends it, then; 2) light the face so it is warm and saturated.
When a highlighted face is put on a dark background it automatically becomes the star in the spotlight by virtue of tonal contrast. All you need to do is aim the key light on the front of the face instead of the side of the head.
On a white background the dynamic is different. It is still contrast with the background which will draw attention to the face, but if the clothing and background are white or very light in tone it will be COLOR contrast that is the attracting force. So whereas on a dark background you want strong highlight on the front of the face on a white one you want the front of the face warmer and more saturated that the sides. That usually happens to some extent automatically in backlight as it did here.
But the goal of the lighting isn't just to help the viewer find the face, you want it to model the 3D shape of the face. The problem is that when you put the sun behind and face the subject into the skylight the skylight is very flat. It's not totally flat because it's always brighter from above than the sides, but the ratio of downward:sideways is so low, under 2:1, that there aren't any "normal" highlight:shadow clue about shape on the face. That can be remedied in that situation with flash.
Here's a shot of my wife I took during a walk in a park. Not a planned photo session, I just had the camera along. I always have a flash on a bracket on my camera, indoors and out. As with your kid she was wearing white so the shot was planned around making it less distracting by finding a light background. As luck would have it the sun was behind bouncing off the river providing ideal rim light, skylighting on the face and a light background. The problem as in your shot was the lighting on the face was flat and the eyes were shaded by the brow a bit due to the high angle. To get the skylight in the eyes I had her stand below me near the water while I was about 3' higher on the bank. It your shot the fact you where taller and he was looking up had the same effect of getting the skylight into those blue eyes. Then to accentuate the lighting pattern the natural light was creating I turned on the flash, knowing the flash on the bracket would create the same pattern, only brighter.
What happens when you add flash and adjust the exposure to keep the now brighter highlights the same is the shadows on the sides of the nose and face get darker which were there but very faint in the natural light get darker and closer to the 3:1 - 4:1 ratio we are more accustomed to seeing faces in. In 2D photos, because all the clues to 3D shape come from highlight:shadow contrast adding the flash pattern over the natural pattern accentuates the natural 3D shape of the face...
So had you shot him on a lighter background with out the green bias and with flash on a bracket to accentuate the modeling on the face it would have looked more like this in the color shot...
It would have looked much better than the edit, which is a quick bandaging job. An ounce of prevention is worth a pound of cure. Mind you there's nothing "wrong" with you shot because there's no such thing as a bad photo of a cute smiling kid, but my edit makes that face more of the star in the spotlight by making it contrast and eliminating the other distractions.