Your point is quite valid, but let's take into account a couple of facts:
1. Virtually all colours can be found in portraits by famous artists, if you look hard enough. A painting which seems to depict a perfectly natural skin colour can turn out all blue when compared to another portrait, painted in a different light. Finding the right colour is seldom about identifying a native pigment - it's always a combination of light reflections and refractions.
2. However, saying "why can't I have green in my portrait when Cezanne can (or whoever)?" is a bit like saying "why can't I paint a perfectly square face when Picasso can?" Because you don't have a valid reason to do so, you just claim it's your right to, and expect that it will somehow magically be appealing.
3. This particular piece looked better without the green. Why? Because the green parts weren't applied in a way that made sense. They didn't improve the face. They didn't accentuate anything.
4. Does this mean it's always wrong to add green in your portraits? Hell no. Look at any recent pixelated portrait by Helm - like this one:

Lots of green. But there is a method to it. It's not like he just threw green pixels at the canvas and hoped for the best. This scene called for the green. It made sense here.
Or why not this one:

Lots of weirds shapes and colours, but Helm had a reason to make it that way, an agenda. It doesn't mean anyone can add lots of purple triangles to their portrait and hope for success.
Why not?
For the same reason you shouldn't begin practising writing by mimicing Finnegan's Wake. It's just no use. Start simple. When you know the basics, start pushing the boundaries. When you can control basic perspective, move on to have a go at DOTT graphics. Etc, etc.
1. Virtually all colours can be found in portraits by famous artists, if you look hard enough. A painting which seems to depict a perfectly natural skin colour can turn out all blue when compared to another portrait, painted in a different light. Finding the right colour is seldom about identifying a native pigment - it's always a combination of light reflections and refractions.
2. However, saying "why can't I have green in my portrait when Cezanne can (or whoever)?" is a bit like saying "why can't I paint a perfectly square face when Picasso can?" Because you don't have a valid reason to do so, you just claim it's your right to, and expect that it will somehow magically be appealing.
3. This particular piece looked better without the green. Why? Because the green parts weren't applied in a way that made sense. They didn't improve the face. They didn't accentuate anything.
4. Does this mean it's always wrong to add green in your portraits? Hell no. Look at any recent pixelated portrait by Helm - like this one:

Lots of green. But there is a method to it. It's not like he just threw green pixels at the canvas and hoped for the best. This scene called for the green. It made sense here.
Or why not this one:

Lots of weirds shapes and colours, but Helm had a reason to make it that way, an agenda. It doesn't mean anyone can add lots of purple triangles to their portrait and hope for success.
Why not?
For the same reason you shouldn't begin practising writing by mimicing Finnegan's Wake. It's just no use. Start simple. When you know the basics, start pushing the boundaries. When you can control basic perspective, move on to have a go at DOTT graphics. Etc, etc.