I'm not sure what I'm seeing.

If the colors of the FIRST 2 pictures are correct, you may have copper staining + plaster mottling.

In the second 2 pictures it looks ONLY like plaster mottling, to me. The fact that you mention "brownish stains" in your first post make me think you ONLY have plaster mottling.

Plaster mottling is a defect in the plaster, possibly accentuated by water conditions. It's NOT a stain. Acid washing will tend to even the surface coloration, but at the cost of a more porous and abrasive exposed surface.

Here are some resources:
http://www.poolhelp.com/handouts/oB_...%20Handout.pdf
http://www.poolhelp.com/handouts/oB_...%20Handout.pdf
http://www.poolhelp.com/handouts/oB_...%20Handout.pdf
http://www.poolhelp.com/handouts/oB_...%20Handout.pdf
http://poolhelp.com/handouts/oB_Conf...%20Handout.pdf
http://www.poolhelp.com/handouts/oB_...%20Handout.pdf

http://www.poolhelp.com/npc_cal-poly.html