You can take an angle grinder to the top layer of the concrete and buzz it out. Then re-mortar it.
It would be almost impossible to temporarily stain the concrete to match the rest of the mortar joints.
As a matter of fact, even when you use the exact same color mortar to the newly fixed bricks it will look slightly off until it has a chance to absorb the dirt and natural patina that the rest of the brick have.

Thank you deviant-deziner (love your name btw). I believe we do have an angle grinder.

How to color match (close proximity) the mortar is another issue. I used to have a book marked site where you could send in a piece of the mortar and they would sell you the right color.

Again, thank you very much.

SparklingWater, can you show a photo of the work? Also, was the original mortar colored, or is it just plain, uncolored mortar? It would be very difficult to use concrete for mortar, since concrete has rocks in it, so what makes you think this is what was done?

If you live near a good tile supply store or a well stocked masonry shop you'll be able to take a pc. of mortar into the shop and they will be able to match the color.
If you are really concerned about the color then after bringing home a box or bag of the mortar I would mix up a small batch and document your exact measurments , do a test slump. let it cure for a day or so and then you'll have a more accurate color sample to work with.

Thank you for your help. Here are two labeled pictures. I think it's knowing cement, not mortar was used and secondly, the "brightness" of the white cement. Maybe I should pour dirt on it, ha. JK.

Brick Walk showing mortar color in cleaner (less dirt) area on right
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Brick Walk showing cement
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CEMENT is the same bonding agent used in concrete and mortar. Concrete has rocks and sand in it. Mortar has sand but not rocks because it must squeeze into tight spaces. The new joint is not filled with concrete, but with new, uncolored mortar. You will never be able to match the existing mortar because it's covered with dark colored dirt and mildew. If you matched the dark color, then when the walk was eventually cleaned or pressure washed, you'd have the same spot being dark while the rest of the walk had light mortar. It would be a losing battle.

If this problem was mine, I'd get some muriatic acid and CAREFULLY clean only the smeared mortar off of the brick. I think this would best be done with a cheap plastic bristle paint brush used to daub acid in the proper spot, since it's such a small area. (If one has never worked with acid before, a person would need a water hose standing nearby ... acid can work fast and sometimes needs to be flushed away in a hurry. Also, one would need to be upwind of it as the fumes are extremely noxious. And, of course, not splash any on one's skin!) I would not use acid to clean any of the mortar that should be there. Acid is too powerful and will eat into it. After the smeared mortar is cleaned away, I'd pressure wash the walk. This will bring the dark stained color to a much lighter uniform state (the brick will look better, too,) making the new mortar not such an obvious difference. Within a relatively short period of time, the natural aging process would obscure the difference even more ... probably to the point that no one would notice it.

Regardless of what you decide to do, you should not try to match dirty mortar. In the long run you would be dissatisfied.

Thanks Yardvaark for educating me about what is in that brick joint. I'm glad to hear it's cement mortar (not concrete), just unmatched to original color. Yes, definitely been a while since a pressure wash has been done on these bricks and the walk needs it.

I have heard of course of muriatic acid although never used it If I can get a very small amount and a helper, I may try your suggestions fully safety gowned and with standby water to clean up the brick.

I may leave this to a professional brick man to fix too. I really was teasing when I said it may be easier to go dark than go light. A good power wash, some touch up brick filling and hopefully removal of the smeared concrete mortar seems to be the best way to go. Then tincture of time.

Thank you very much for your help.