The color of the stains looks to be a derivation of the original wall color, with the edges being slightly different than the center of the stain. Fresh stains are darker towards the center whereas old stains have lifted the color from the center of the stain and the center is less saturated.
The shape of the drips are controlled by a noise stretched in the Y direction. Roughness controls the amount and frequency of the smaller drips.
The color is based on the original diffuse of the wall, with changes in the hue, saturation, and value. The user also has the ability to make the center or edges of the stain a different HSV value. This is based on the original intensity of the noise because the center of a noise patch is generally higher in value than the edges.
Now I need to python script the ability to paint attributes so the user doesn't have to set/mess that up, also I need to implement the water stains within the cracks. I think the color attribute from the original water stain will be suitable to control that.