Each time I paint, I transfer my used solvent into my 'sludge jar'. The pigment in the used solvent settles to the bottom of the jar, forming a neutral paint sludge. I then tip my clear, used solvent into my 'painting jar' and repeat the process each time I paint. Eventually, the jar fills up with a neutral grey paint sludge made up of my color palette.

You can recycle the sludge and make your own neutral grey paint.
I will use this paint to grey-out flesh tones, etc. in my future paintings.