I've been quite frustrated over time when I've returned to the canvas I'm working on to find that an area that had looked dark has dried without luster and looks much lighter.
I paint on linen canvas sized with "rabbit skin" glue, grounded with lead white and then a Neapolitan dark brown mixture of oil pigments, but this has happened with other grounds too.
I've heard that this means that the color is "sinking" into the canvas. Sometimes oiling it out with some straight linseed oil (if that's the medium I'm using) or a mixture of linseed oil and mineral spirits (Grumtine or Best Klean) will bring back the shine and hence the darkness/transparency, but not always. Even retouch varnish doesn't always help in the worst areas.
I've noticed that if I use more medium (linseed oil in my current painting), the dark colors dry shinier, hence darker.
Also, is there any way to get my lights (pigments mixed with Cremnitz or Flake w/ Mica) to the same luster as my dark, transparent colors?
Deborah Feller, MFA