Is it normal for my patio concrete to be turning grey so quickly?

That uniform grey look you are seeing is almost certainly the normal result of the hydration and curing process and likely not a mistake in the proportions. As the cement reacts with the water in the mix to harden, the surface actually absorbs ambient dust and atmospheric elements, which gives it that deeper, more consistent grey tone over the first few days. It is simply developing its own patina, which is typical for most exterior residential pours. You are definitely not seeing a problem with the mix itself. Most people worry about this color change, but it is expected when a slab is reaching full strength. The moisture is migrating to the top, and that surface darkening is the cement performing its final steps of setting. If you were aiming for a much lighter, sandy color, that might require specific pigments to be added during the initial batching, which is something that has to be sorted out before the pour happens. But generally, this shade is the standard outcome for this type of concrete.

If you are strongly set on a different look, you must wait until the slab is completely cured, usually several weeks, before attempting any surface treatments. A good sealant can either intensify that grey or introduce a slight color shift, but trying to scrub or scrub it down right now will just create streaks and uneven marks. Just let it sit and cure; it will settle into its final appearance.

Same thing happened with the walkway I did at my old house. It went dark grey for the first week or so and I panicked that I'd botched the water amount. Then over the next month it slowly evened out and looked way better than I expected. One thing I did learn the hard way, don't walk on it more than you have to early on, my footprints stayed visible for a while. Did yours dry with any lighter or darker patches where the sun hits differently, or is it all one even shade?

The other thing nobody thinks about until it happens is water on the surface early on. I had a sprinkler clip the edge of my shed pad the day after pouring and that spot stayed darker than the rest for a whole summer. If anything drips or splashes on yours, same deal. Also if you want it to cure slow and strong, throw a tarp or some straw over it and keep it damp for a few days. Mine came out noticeably less dusty that way. What's the weather been like since you poured? Hot sun on fresh concrete makes the color go all kinds of weird too.

One thing that messes with the color is when the finishers get on it too early and work the bleed water back into the surface. That'll make it darker and kinda blotchy for a while. If they did a broom finish vs a troweled one, that changes the look too. I'd give it a good month before judging it. What did they finish it with, broom or smooth?