I sprayed 40 something pieces of Walthers street system sidewalks this morning only to find out i had a bad bottle of Flowquil concrete paint. So I lightly sanded it smooth and now plan on repainting them. I am not sold on the concrete color though. Most of the cement sidewalks I’ve seen are more of a grayish color. In short what color(s) do you use.
I use Polly Scale “aged concrete”. Then dry brush it very lightly with black weathing powder. To me it looks like concrete and does kinda have the “grayish color” {due to the black powder} you might be looking for.
For my roads, I use “Gray” acrylic craft paint. For the sidewalks, I use a slightly lighter shade. The budget paint brand called that one “Rainy Day Gray,” but any slightly lighter paint would do.
I picked up a quart of “Stamped Concrete” at Sherwin Williams one day. Figured that would last me a zillion years. 
BTW, the color is excellent. I went out looking at sidewalks one day and like someone else pointed out, concrete (at least the stuff around here) is grayish. Not quite the gray of older asphalt but it’s not far from that.
Concrete Color
Concrete has various colors in nature. After much experimenting to get the right concrete color I settled on these two for all my concrete. Delta Ceramcoat Sanstone diluted to your taste with White Ceramcoat for walls and bridges where the elements have had time to work on it.
I use Delta Ceramcoat Mudstone for station platforms, inspection pits, and turntables etc. where oil and dirt have built up over the years. You can dilute with white paint.
A wash on top of these base coats using India Ink or diluted (browns, blacks or grays) paints or caulk should give you the results you want but you have to experiment. The object is to
I mix a little Apple Barrel pewter gray into some plaster of paris. After the pour is dry, I smooth it, scribe some lines on it, and hit it with a wash of india ink and alcohol.
Tom
I use Model Master Light Gray pn FS36495. After I paint it, I take a Crayola black pencil and lightly draw in some cracks.
As Doc mentioned a light India Ink wash really improves the look. For light grays I use a mix of 1 tsp ink/pint of alcohol.
Lance
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I take my Walthers street pieces (and streets and sidewalks I make from styrene) outside to paint with hand held spray cans. I have found that holding a can in each hand of two complimentary but contrasting colors gets the correct slightly varied look to the streets and sidewalks. I show this “two fisted” technique in one of my Frugal Modeler columns in the Midwest Region’s Waybill: http://www.mwr-nmra.org/region/waybill/waybill20093fall.pdf
For colors I like a tan/gray combo. I have found that Tamiya spray can TS-68 Wooden Deck Tan or AS-15 USAF tan contrasts well with their AS-16 USAF light gray. Applied with care the Tamiya paints provide good coverage, no splatter, and a nice flat, and slightly rough, finished texture that totally kills all plastic sheen. Depending on the amount of tan in the mix, this mix enables me to use the Testors’s paint stick weathering pen “Aged Concrete” found in set F3802 for touch up and any edges that avoided getting fully painted. The Testors pen is more on the tan side.
The Tamiya spray cans are not cheap but I like the quality. I follow two rules - first I really do shake them up well before hand, never less than a minute and a half of shaking, and ideally a full three minutes. And second, yes I do hold them upside down and fully clean out the nozzle after use. This “wastes paint” but not nearly as much as having the nozzle become useless due to clogging.
Dave Nelson