It's a weird thing when you think about it. If four color decks were the "standard" only a heretic would suggest cutting down to two colors.
Came here to echo ^this^ sentiment. 4-color is all I use online, but I find them distracting in live play (and helpful to lesser-skilled or inattentive players).Great on the computer, but I don't love it live for some reason.
As the resident old, blind person: I have yet to see a four-color deck in which the green and blue cards are anywhere near as readable as the black and red at any distance greater than arm's length.