Chives are an herb, green onions are part of...well an onion. They are often used in similar ways and in similar dishes. You can substitute one for the others in most cases, but I could not say they are interchangeable. They have distinct flavors and it will change the dish to change the ingredient.
Also do not attempt to replace green onion with dried chives 1-to-1 (or fresh chives for that matter). Dried chives, not uncommonly found in pre-packaged spice racks, have about as much flavor as straw and will not deliver the desired result (unless you like straw).
The difference for green onions will be most noticable if you don't cook them. Instead, slice them up and use them for a garnish over the dish.
The white portion of the green onion will still be oniony, but not quite as strong as a raw red onion. Slice them thinly and use for garnish if you really want to bring out the onion qualities of them. If you don't want it too strong, then add them when you're cooking. (and you might not want to slice them as thin).
The green portion gives a more mild onion taste but also some of the grassy quality that you might get from chives. I treat them like a fresh herb, chop them up, and add in them in the last minute of cooking or I don't cook them at all.
As for substitution ... I'd only plan to do it if you were cooking for someone who didn't like onions, as they're milder and come in smaller portions than bulb onions, so it's easier to control amounts without any waste. ... and I'd do it if I had them on hand but didn't have bulb onions around.
If you're just looking for a good way to feature the green onions -- grill them. Trim the ends off, clean off any dirt, give them a coat of oil, then toss them on a hot pan or grill. They make a great side dish.
Other good uses are to add them into scrambled eggs, green onion pancakes, or press them into naan before cooking it.
...
For the white onions, the opposite is true -- they're generally stronger than red onions. Most people don't serve it as a raw garnish unless it's very thinly sliced and used in small amounts. It's still used raw, but it tends to be blended into other things such as pico de gallo, or guacamole, where the pungency of the onion helps to balance out the sweetness of the tomatoes or richness of the avocado.
I'm having difficulty thinking of a dish where the qualities of the white onion would stand out, though. My only idea is french onion soup, because it just comes out lacking if you make it with sweeter red or yellow onions ... but unless you made them side by side and compared them, I don't know that you'd see the difference.
You might try cooking them slowly over medium heat until they caramelize ... it'd let you feature the onions without them being overwhelming, and it brings out some of their more interesting qualities.
Best Answer
If you want to be really, really picky, then the 'green' is each individual leaf as it breaks away from the cylindrical whole.
That leaves you with the decision as to whether to pick off each leaf, giving you a bit of extra 'white' or just make some relatively arbitrary decision after you hit that first leaf 'node'.
So, unless you want to pick off each leaf, then it's 'about here… ish… '
Personally, if I'm being 'a bit picky' I will unwrap the outer layer for the first couple of nodes, where they go unattractively lumpy, purely for aesthetic reasons & include the rest in my 'white' distinction. Once I get far enough I [that I can't be bothered any further] then I pick out any remaining 'white' bits & go all-in for the rest of the greens, until I hit the raggy ends, which join the roots in the compost bag.
Once you've actually cut to the green line, separation for 'fussy distinction' reasons does get easier, as you're no longer dealing with the entirety of the onion & its reluctance to part layers without ripping down to the root - you can just pull bits off & deal with them separately.