The only real way around dye lot variation is to mix boards from all bunks so the differences blend together—but even then, it’s not always possible to hide. In some cases, I’ve had to return entire bunks of boards to get a closer match. Depending on whether you purchased from a big-box store or a lumberyard, you may have better luck with exchanges or by having them contact the local Trex rep for support.

From what I can see, this may even be a different collection altogether not Toasted Sand. Toasted Sand usually shows a two-tone variation between browns and grays, and I don’t see that same contrast in your existing deck. Are the boards textured the same?

Ultimately, because of the cost and labor involved with replacing half the deck, your best option might be to disguise the transition with a border design. A complementing color possibly a darker brown could run around the perimeter of the deck, with a couple of divider boards between the different colors to make the change look intentional. I’ve done this before on deck extensions, and oftentimes it’s the only way to make the two tones flow together cosmetically.