Welcome to RP!
What I did to remove the cyan stain from the house in your picture is the following:
- Activated the Blue Channel. (the one with most details and where the stain was less evident)
- Copied and pasted the selection on Green and Red Channels. (now the house was gray..)
- Used the Levels on Red and Green Channels to restore the colour of the house.
- Used the Patch Tool on the 'flat' area of the house just above the roof.
You might find this Tutorial
, which Leah kindly pointed out, very interesting ....