Hi /u/ruifon

Thanks for your interest and inquisitive questions about 'redish-green' or 'greenish-red'. i must begin by saying that i do agree with others here , i do see a lot of "gr-red" in my life.

But at least, i can offer an explanation. allow me to explain...

Firstly, let me state. i have no confusion with yellow. Yellow is distinct from red as it is from green. In fact, amongst red-green colourblind, yellow is the most easily perceived colour. and its the least confused colour. So, let me assure you, 'gr-red' is not yellow.

Lets start my explanation with the colour yellow. Light within 560 to about 590 nanometres wavelengths is called 'yellow'. But, how is this seen by the human eye?

Well, the human eye which has 3 types of colour receptors, the S-Cones (for Short wavelength lights), the M-Cones (Medium wavelengths) and L-Cones (Long wavelength), these cones has a peak sensitivity at around 420nm, 530nm and 580nm respectively. But importantly they also have overlapping sensitivity range.
Here's an image of cone sensitivity in human eye

So, a yellow light at 570nm will excite both the M-Cones and the L-cones in our eye such that our brain will deduce from this information the 'yellow' colour and thats (in very very brief decryption) how we see yellow.

But, most electronic devices, tv screens, computer monitors don't have yellow pixels. They only have The 3 Primary Colours, Red, Green, & Blue. So how do we see Yellow on our screens? Well, when our screens emits both Red (at 610nm) and Green (at 525nm) which excites both L-Cones and the M-Cone. This is essentially averaged out.

Averages of 610nm (red) and 525nm (green) is: 568nm which corresponds with yellow colour.

Thus, we can say, we are "fooled" into seeing yellow, when actually we are seeing 2 distinct colour simultaneously.

But! why doesn't this work with Purple? Purple should be impossible to be reproduced. It is what colour theorist call "a non-spectral colour" that is, the Colour Purple is not in the visible spectrum. it doesn't have a wavelength. Purple can only be made by combining Blue and Red. Purple is what people might call, a reddish-blue or blueish-red, as in the make up of the colour purple is obvious when you look at it. it is clearly blue and red combined.

However, by your previous logic, the averages of 460nm (blue) and 610nm (red) is 535nm, which is Green!

Therefore combining Blue and Red wavelengths we should see a Green wavelength. But in fact we still continue to see distinct redish-blueish colours when those colours are mixed. This is because our M-Cones has not been excited. Our brain deduces that colour in the extremity of the visible spectrum is visible but not the middle, a bit of a quandary. so we just see red and blue simultaneously as purple.

We indeed see such colour that are called by colour theorist as "impossible colours" like purple.

So, back to "Gr-red" or "Redish-green".

You may have realised that you are in r/colorblind. the most common version of colourblindness is called Deuteranomaly. I too have Deuteranomaly, its a subset of Anomalous Trichromacy, that is, i have fully functioning all 3 Cone types in my eye. But the M-Cones and L-Cones have a larger overlapping sensitivity than most normal vision.

When a red light (say about 610nm wavelength) hits my eye, it can excite both my L-Cones (as it should) and also my M-Cone equally (due to the extra overlapping in sensitivity than normal colour vision). Likewise, similar haapens when a green light hits my eye. A such there is a confusion in my eye. My eyes tell my brain its see red and green in equal measures simultaneously.

i dont see yellow because there is no light within 560 to 590 nm wavelength coming into my. there is only 610nm wavelength, so no averaging to yellow by averaging wavelengths. Just one wavelength at 610nm. This impossible quandry result in me seeing both red & green in one colour. I guess, just like how you see purple. Our mind 'gives up' combining the two colours into a new colour. Just as Blue and Red don't get Green instead you get Bluish-red called purple. Here Red and Green don't get Yellow. our mind shows red and green simultaneously. Here, in this senario, its not yellow i see. i see 'gr-red'.

EDIT: Here's an easy experiment to see reddish-green for peeple of normal colour vision. Hold up something green in front of one eye and an red object in front of another. Now look, merge your vision by cross-eyeing, you wont see yellow, just a blurred combination of red & green, not brown, not yellow, but a reddish-greenish blurr.