There is an axiom upon which there appear to be different variants:

"Red and green should never be seen except upon an Irish queen"

"Red and green should never be seen, without a colour in between"

"Red and green should never be seen, except on an Irish Colleen"

Then there is also a version about red and green navigation lights on ships to avoid collisions...!

Personally I think that red and green do look nice together!

And a lot of tartans have red and green in them anyway!