In the Northeast

[doctorwayne]

b. The silo was white painted but the concrete is very visible at a lot of small spots
That would be white and concrete color (PollyS)

c. The silo is not painted at all. It is concrete color with weathering
That would be concrete (PollyS) and aged concrete (PollyS)

I prefer B but have no clue how to do that. May be similar to dry brushing with concrete color over white. Masking tape could be used to enforce horizontal pattern.
Second choice is C.

For B, I'd paint the silo with the concrete colour first, letting it cure completely. Then you have at least two choices: you could paint over the entire structure with white, and then lightly sand it in some spots, or you could paint the concrete colour, let it cure, then randomly daub it with rubber cement or contact cement. Next, paint the white, then use an eraser to lift the contact cement, revealing the concrete colour.

If you're going to use paint as the weathering agent, try using a darkened version of the base colour as the primary weathering. For instance, paint the silos with "Aged Concrete", then use the same colour darkened with some brown for the weathering - this would work for the "ring effect" with the masking tape. If you want to add other effects, such as rust stains, do so after the initial weathering, then take either your darkened base colour or a mixture of the darkened colour and the original base colour, thin it with about 90% thinner, and overspray the entire structure. This will tone down the weathering effects, blending them together in a subtle manner.

Wayne