Maybe it was mentioned over a hundred of times, but I didn´t get the different color of Klingon blood. Sometimes it is pink/margenta (TUC Klingongs), sometimes it is red (Generations, Duras sisters).
Klingon blood is red in all but TUC. The only reason it was magenta there is that it would have bumped up the movie's rating. I guess since the pink blood was the contradiction, it was decided not to amend Klingon blood to that colour for any future productions. Larry Nemecek posited, "Perhaps, in hindsight, the blood globs we saw in ST:VI were interacting in an odd and visible way amid toxins in the malfunctioning atmospheric system of Gorkon's heavily damaged ship!"
Actually, strangely enough, Broken Bow does feature a computer graphic of Klingon DNA which includes purple blood.
Though really, the only reason they made it purple in TUC was keep the movie PG. If it had been red blood floating around the movie would have gotten a PG-13 rating.