A contractor installed a cellar door for me recently and used white concrete/cement to bed it in. This contrasts badly with the surrounding normal (gray) concrete apron so I'd like to change the color obviously without pulling it up or waiting 20 years for it to age naturally.
Since you say cellar the old is dirty and moldy, you could clean the old easily, laundry bleach will kill mold in minutes, I use a garden sprayer. You could then powerwash the old and use Muiatic acid if you want it real new but bleach often works. If it still doesnt match rub dirt in the new. What he put in is grey with grey portland, White Portland and White sand and gravel would be expensive and made on site by component.
A neighbor in our condo stained some furniture on the outdoor concrete deck in our condo atrium. When the condo board started to rumble about it, I got out my craft paints and painted out the dark brown stains. This was about 6 years ago and it is still good. You can get small bottles of acryllic craft paint at Michaels or Joann. Mix a black, white, a little dark brown, a touch of blue and yellow. I used two or three shades, as in "faux finishing" to give the painted area more of the texture of concrete. Use a round stencil brush that is flat on the end, squeeze out excess and dab on thin coats. Nothing to it, and a heck of a lot easier than messing with mortar.
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