It seems to be very difficult to get a definitive answer to the red hair and blue eyes versus red hair and brown eyes question through googling - the red hair and blue eyes being the most rare combination seems to be coming from one source that all the other sources are quoting. It seems to be based on a statistical thing where red hair is very rare based on world population, and blue eyes are quite rare based on world population, so, based on world population, having both is extremely rare.
But, red haired people tend to be in populations where light coloured eyes are more common, and in these populations red hair and blue eyes are not rare. I saw an analogy on Quora - tall people who speak Serbo-Croat are very rare in terms of world population, but not very rare in Croatia.
Wikipedia (I understand can't always be trusted) suggests red haired people usually have light coloured eyes and gives a scientific article as a reference - I didn't read it as it was behind a paywall.
It has definitely been my experience that red hair and brown eyes is unusual, but obviously that's just my experience.
There is also a link on Quora to a Facebook survey. Blue seem to be the most common.
www.quora.com/What-color-eyes-do-most-redheads-have-Why