Yesterday I used bright colours to grey down a work in progress.
The canvases were also bright colours to begin with, as you can see by the edges which I did not paint over. One bright colour painted over another can make for some very lovely off-tones.
I'll be working over them again with more bright colours in sheer layers, to get them really neutral, close to greys. In this way the colour will remain luminous.
Then I used those same bright colours to work over a red underpainting, with good results.
And I discovered this photo I took when I removed my Fragile Planets installation from the Drumheller gallery. There are no bright colours visible in this photo... but in the polished concrete floor are all sorts of colours which blend into an overall neutral grey.
If the baseboards were painted in pure bright colours, this floor would be rather like my envisioned paintings. :)