I have a number of different types of stains on my plaster, and I'm trying to diagnose what they are. Some of the stains were there when we bought the house, so I'm not sure what the previous owners did or did not do to cause it.
Light blue - large parts of the plaster are blue and splotchy. It looks like it's actually a part of the plaster to me, so maybe just a faulty plaster job?
Grey - patches of it are grey and splotchy. Looks very similar to the blue splotchy patches, except for the color.
Brown - this is relatively recent. I've been fighting off what appears to be pollen accumulating on the bottom of the pool. In certain spots it appears to have gotten into cracks and I can't seem to get it out. I tried the ascorbic acid test with no luck, so the stains don't appear to be metallic. Notice that there's also some grey in this picture.