That was the first alarm bell that went off for me, personally.
I conceived on April 8th or so, and by April 20th I noticed that my nipples were significantly darker. They're normally a very light pink, but the were brownish in color toward the end of April. Also, my boobs seemed GARGANTUAN!
This prompted me to take a pregnancy test a few days later, which of course was positive.
Of course, everyone's different. Also, it sometimes depends on your original nipple pigmentation.
Currently 36 weeks pregnant.