I have light-brown eggs to deal with when I have to candle. I have yet to EVER see a "vein" anywhere, and I've used several kinds of lights. All I can tell you is what gives me a clue about what's going on in the egg that Miss Prissy told me:
On day 14 If there's a black blob showing through the egg, a chicken is forming. If the egg is clear and practically glows, it probably ain't never gonna produce no chick.