The old fashion advice is 'red and green should never be seen' :D
They're basically Christmas colours.
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https://www.extratime.com/articles/1...1-mayo-league/
This suggests they added red between 2016 and 2017.
I guess if there was an incentive to join an Intermediate League clubs would do it - so as you say if the FAI just announced that to enter senior League of Ireland you had to be promoted via a "Senior League" it wouldn't be long before a Connacht/Ulster one was established and clubs from outside Cork would join the MSL.
Er, don't you mean: "Blue and Green should never be seen, without a colour in between"? (Google it).
Anyhow, there is nothing wrong with Red and Green, so long as it goes with Black:
https://www.glentoran.com/sites/defa...colourised.jpg
This is what the BBC says in Are there really 'rules' to what to wear? - BBC News :
"Then there is red and green which "should only be seen upon an Irish queen" and "never be seen without a colour in between", according to traditional sayings."Often these rules come from how similar in tone and hue the colours are supposed to be," says Butchart. "
The red and green thing is because wearers look like one of Santa's helpers or a Christmas tree in those colours. Blue and greeen is actually fairly common - including in a number of N.Ireland jersies in recent years.
CULT KITS - 2016/17 NORTHERN IRELAND HOME SHIRT (M) ADIDAS – Cult Kits
Calling it now: Jersey colours are the new league pyramids!