It happened in the past and now again. The buttons on applictions caption bar (minimize, close, smaller) turned green. These are the buttons on the top,right of the application window.

What can be done?

penicillin?

It's the RB# virus.

Shut off your computer. Unplug it. Remove all the cables - monitor, keyboard, mouse, printer, ethernet, etc. Fill the bathtub a mixture of 2 parts water, 1 part bleach, and 3 parts salicylic acid. Douse the computer in the bathtub and let sit for 2-3 hours. Rinse clean with water and dry thoroughly.

When you reassemble your computer, you will no longer see green buttons.

It's better to use Picric acid that's been sitting around for 30 years.  You get a much bigger boom, thus ensuring the problem is gone for good.

Maybe it's actually a pin on the monitor cable that's gone faulty? Is everything tinted green?

I had something like this when a poorly written app (mine) clobbered bits of the Windows palette.