Start with the shape of the pattern
Before the cause, look at how the two colors meet. Soft, cloudy patches that fade into each other, dark in one area and pale a few feet away with no clean edge between them, are a curing pattern inside a single pour. That is the one this page is about. A hard straight line where one shade stops and another starts is usually something else, and it is worth sorting out which you have before you go looking for a reason.
- Soft, cloudy patches with no clean edge: a curing pattern inside one pour
- A straight edge with older, grayer concrete on one side: two ages of concrete meeting, which is what a patch or a replaced section looks like
- A white chalky film that wipes off on a dry finger: efflorescence, a different problem with its own answer
Where the slab lost its water decides the shade
Concrete takes its final color while it is curing, in the first days when the cement and the water in the top layer are still reacting. Anything that makes one part of the slab give up moisture faster than another part shows up later as a shade difference. Afternoon sun on half a driveway and shade from the house over the other half will do it. So will wind crossing an open stretch while a corner sits still behind a truck.
How the slab was protected matters as much as the weather it sat in. Plastic sheeting cures the concrete it lies flat against differently from the concrete it tents over, so wrinkles in the sheet can print themselves onto the surface as a mottled pattern. A curing compound sprayed heavier on one pass than the next holds moisture in unevenly for the same reason. Neither one is carelessness exactly. They are the marks of protecting a fresh slab in real wind on a real job, and they are why blotching so often has a shape to it instead of being scattered at random.
The ground underneath is the third piece. A subgrade soaked in one area and dry in another draws water out of the bottom of the slab at two different rates, and that reaches the surface as color the same way weather at the top does.
Two loads in one pour can read as two shades
A pour big enough to take more than one truck sometimes finishes with a faint band across it where the loads met. Two loads out of the same batch plant on the same morning are mixed to the same design, not to the same shade. The cement production run, how much water the sand and stone were already carrying that day, and how long a load turned in the drum before it went down all move the color a little.
A load that shows up stiff and warm moves that further. A hot load, one that has started to set before it is placed, gets pushed, screeded and floated differently than the easy load behind it, and concrete worked differently often finishes a shade off. That is a timing story, not a quality one, and it is why the band tends to sit right about where one truck ran out.
Finishing pressure changes how the surface throws light
How hard and how late a surface got worked changes its color too. Steel troweling packs the top layer tighter, and a denser surface reads darker. On a garage or shop floor that gets a power trowel, a section that took an extra pass, or took its passes later than the rest of the floor, can come out noticeably darker than the bay beside it. On a broom finished driveway the same effect is softer, because the texture a broom leaves scatters light instead of reflecting it off a polished face.
That is also why the pattern can look strong from the porch and nearly gone from the far end of the driveway, and why it changes through the day as the sun moves. Some of what you are seeing is sheen rather than color.
Why it is cosmetic, and what actually evens it out
None of that touches strength. A curing pattern lives in the top fraction of an inch, set while the surface was still finishing, and the concrete under it was placed, consolidated and cured as one slab. Color tells you about the surface. Strength comes from the mix, the base and the cure through the full thickness, and a slab can be blotchy and sound just as easily as it can be perfectly even and poorly supported.
Time is the real remedy. Once the whole surface is taking the same sun, rain and traffic, the light areas and the dark areas move toward each other, and most one pour blotching is much harder to see after a couple of seasons than it was the week the forms came off. Wash the slab before you judge it, since a film of dust exaggerates a pattern that is already there, and look at it dry rather than in low evening sun.
What will not help is a heavy film sealer put down to hide it. That darkens everything under it and adds a gloss that can make the pattern easier to pick out rather than harder. If the color still bothers you after a season or two, changing it is a cosmetic decision with its own options, not a repair the slab needs.