I find myself corrected: From Season Three on (after "Elaan of Troyius") the door opposite to the transporter room is red.
I don't think "no thought process" is fair. Take "Amok Time" where at the beginning McCoy is leaving some room at the end of Spock's corridor which usually is a turbolift (with red doors). For this particular scene the door color changed to grey.
And for the signs adjacent to the doors they mostly made an effort, especially in Season One, to change the texts: "Personnel Director", "Science Library", "Computer Systems", "Briefing Room 2", "Astro-Medicine Ward 1" (aka Sickbay) etc.
In the beginning, they tried to mark crew's cabins with black signs and others rooms with blue signs.
One of the signs they appear to have used permanently is a blue one saying "Officers Quarters" with an arrow and cabin numbers.