Can I modify pigment of dried paint on wood?

[Michaelangelo]

I have a wood ceiling that was lightly "washed" with a dilute turquoise paint (could be water or milk-based). I'd really like to tone down the blue without smearing, removing, or adding paint. Do chemicals exist that would attack the pigment, but not the binder? I can test in a hidden area.

[HydrogenBond]

You can try oxalic acid, which is a wood bleach. This is used to get the gray out of weathered wood and might help weaken the color. Regular bleach tends to make the wood look unnatural and should not be used. Oxalic acid keeps the wood natural.

 

Also the opposite of blue, using paint pigments is orange. So if you used a watered down cedar (orange) color, it will cancel some of the blue and make it more grayed off. The opposite of green is red, which is one of the ingredients in orange.

 

If you started with green pressured treated lumber, since the opposite of green is red, one can cancel the green color. If one wanted it to be brown, one would use a reddish brown to cancel the green and leave the brown. Depending on your goal there is always a way to add colors to get the result.