Is this a Plaster Bridge completely scratch built?-if so-GREAT JOB!

As to painting and weathering-hope this helps:

Best idea would be to use some sort of a reference shot/bridge that would be similar to what you are building. (google image search for bridge can help with that) As to the weathering-I use a number of things, a wash to dirty up the rocks and some pigment powder mixed in with thinner and sand (aim for the "in between the rocks/bricks space).

Basically, you'll give it a gray base coat. Then mix a bit of white or black into the gray and vary the color a bit, and paint individual rocks. Do this so there is apparantly random coloring.
Then go back in and mix a dark mortar color and do the mortar lines.
Lastly give it a wash of black/burnt umber, concentrate on weather lines, where rain and such drain.
You could also wash a dark greenish color around the opening to represent algea.

Here's another technique, it's the one I've used in the past and have gotten good comments on them. Once, I even got asked if I used real stones.

I start out painting with a dark grey to black base coat. I treat each rock as a seperate model. Some rocks I use shades of grey, going from dark to light. Others I use red-browns. In all cases I dry brush each color slowly building into a real light color.

Rocks that wold be normally expsoed to water get some coats of dark green (algee color: spelling?) I also apply coats of clear gloss to represent wet spots, etc.