Like all the trees in a forest lean in a given direction, all carpet nap leans in a given direction too. It came off the loom that way at the Carpet Mill.

If your carpet seems to be a slightly different color on either side of a seam, it may be because the nap is leaning one way on one side of the seam and leaning the other way on the other side of the seam. It could be an ‘A’ seam but the color variation is totally obvious. It's the nap laying in different directions!

It’s not your job as the customer to know. It’s your job is to complain about it. A good installer will get it fixed for you. And if it’s his fault, he’ll fess up. However, if it’s a color mis-match issue, the manufacturer ought to step up and replace it. Either way, the installer is the most qualified to make the call. If he runs from the problem, that’s a problem. If he tries to resolve it, be as patient as you can. These things happen. Like they say, we all have problems, it’s how you handle them that matters.

To satisfy your own curiosity, you can try the paper and pencil test to confirm that it may be turned:

All you do is lay a sheet of paper (81/2” x 11”) flat on the carpet (one side of the seam at a time) then lay a long pencil flat on top of the paper. Now gently roll the pencil back & forth with the back of your hand and watch for the paper to move. Fluff up the nap and repeat the test making note as to the direction of motion of the paper. The paper will move in the direction that the nap leans. Mark an arrow on the paper in the direction that it moves. Then repeat the test on the other side of the seam.
It takes some practice, but you should see the paper moving in the same direction on both sides of the seam. If not, it’s been ‘turned.’ And that’s installer error, plain and simple!

Again, it’s not your problem. Just point it out and hold-on to your money until it’s fixed. Especially if it’s a retail installation. Have a witness (not a friend or relative) confirm your view and get it in writing.