How do I make new wood and old wood on a deck look the same?

Multiple coats.

You could always just paint it over, old and new. I debated this myself about 7 years ago and went ahead and painted the whole decking a super natural looking sort of sage green, It turned out beautifully!

I can think of two ways: you can rent a belt sander and take the patina off of the old wood and you can use some vinegar on the new wood. It’ll make it look grey like aged wood. I would take a couple of scrap pieces of the new stuff and experiment with the vinegar and see if you can’t get it to look about right. You really should let the new pressure treated wood age a little before you paint it if that is ultimately what you end up doing so keep that in mind.

Here’s a video on the subject. The guy has a cross beam in his cathedral ceiling a portion of which he has had to replace.

@ARE_you_kidding_me
The old wood really needs to be resealed and stained again. That is why I have to replace some spots. I’m afraid if I wait doing that, in a few months there will be more planks lost. I thought of the vinegar but will it hurt chemically treated wood? Would I have to add a water seal after it? Chemically treated wood is used to keep the wood from rotting for a few years but usually takes a few months before it can be painted because it repels paint and stains till its been exposed to the outdoor for a few months.
@Coloma, did you use pressure treated wood / chemically treated?

@Pandora Originally it was yes, but I just had it lightly sanded and primered and it took the paint beautifully.

Just don’t reseal the new parts yet but that may be easier said than done.

SAND IT DOWN. Sand both of them down to smooth. Smooth enough that a 2 year old could run and play and slide and be ok on this wood haha
After that, you could stain it. The stain, however, will change the color, not the tone. So if you want them to look the same, paint it.