Possible solution to getting realistic concrete

Small containers of Pour Rock or the Sacrete equivalent may work for you, I never experimented w/ this method but let us know your results.

I personally like to work w/ Hydrocal tinted to approximate coloring w/ masonry dyes. To me, the Hydrocal castings look the closet to to concrete after weathering w/ various washes.The dried casting can be distressed, chipped and gouged for realism before and/ or after assy.

These abutment pieces done by a club member by this method

The walls and capping dyes and washes to match the modified WS widened portal

For painting, many time concrete is assumed to be gray, actually old concrete is more of a dirty stained tan. You can start w/ a base of light greay and color according to how much weathering (age/ dirt and stain) you want