What I Use

I use various colors for concrete.  I don't like having to mix if I don't have to, so have found several colors that work straight from the bottle or can.  Try Testors Model Master "Camouflage Gray;" it's a brownish-gray color.  I also like Design Master "Stone Gray" spray paint that I picked up at Hobby Lobby, which is somewhat darker.  Another option is an "almond" color, and there are numerous spray cans of that kind of thing from different sources.  To use the spray can paints for brushing window sills and such, I spray a small amount into a spare bottle or paper cup.

Concrete weathers like anything else.

 think the aggregate color will also change the shade of the concrete mix.  In addition, the owner of a building or the architect may have spec'd a die color for the concrete before the pour.  There may be other factors that I'm not thinking of right now.  The point is that I don't think that there is ONE concrete color.  You need to try to color match your prototype, or just model "close enough." 

Foundation by Testors

You mention a window sill which is a fine aggregate type cement.   I have used Testors - Foundation- enamel,

or try mixing  sand, flat or antique white and some light gray paints together until it works for you.

James

Limestone sills

If the building is older than the 1980's, the window sills are limestone, not concrete.  Limestone has that creamy color if from Indiana quarries.  Alabama limestone has a slight dark veining, similar (but much less intense) to marble.  Modern buildings have precast concrete, or similar composite material, sills, tinted to simulate limestone color.

Concrete Pigment

I have had some pleasing results using Woodland Scenics Concrete Pigment weathered with an India Ink wash.

As Russ mentioned though the color varies greatly so there is likely no "right" color out of a can, for all instances.

lots of colors for concrete

Pick a light grey, beige, light yellow or somewhere in between or there abouts.

I used to travel in a previous life and over time noted that concrete color varies quite a bit across the country.  The yellowish/brownish seemed to be in New England and the greyish on the west coast.

Mine is grey here in Klamath Falls and is about 5 years old.

Terry

gray and wash

I have used several grays on tunnel portals. Let it dry and then make a thin wash of a brown, various browns will work. darker for older. Use this wash and it will add a touch of tint to the concrete and age it.

Ive also used a thin wash of a green to tint some bridge piers.

Once dry, use some chalk to add a few dabs of yellows, whites, greens, blues or rust. I use charcoal to add smoke on portals.

Am I missing something as..............

..................both Floquil and Polyscale come in concrete colors that look correct to me!?  A little weathering with lighter and darker pigments (dry) will add highlights to give it "character".

Peter

Polyscale

I use polyscale aged concrete (because that's what I have) and an india ink wash.

the aged concrete by itself

and after the ink wash.

Not perfect - maybe, but as the scene gets nearer and nearer to completion

it begins to disappear more, looks appropriate.

Looks real....

..........to me.  Nice job on the overall scene Scarpia.

Peter