I was unaware of this, because for me, all levels are always shown in green. I see a menu with items like:
023: LEVEL 23
024: LEVEL 24
Levels completed will be in all green. Levels started started but never completed will be in all yellow. and Levels never started will be in all red. [at least for me]
Rocks'n'Diamonds main menu > click on on numbers on the right of 'LEVELSET'
What 'CUSTOM GRAPHICS' theme are you using? Maybe it has something to do with this?
Rocks'n'Diamonds Main menu > 'SETUP' > 'CUSTOM ARTWORK' > 'CUSTOM GRAPHICS'.
I'm using The 'WIDESCREEN MODE [1920x1080]' theme I found in these forums, but the level history based coloring also works this way when I switch it to 'CLASSIC' or 'CLASSIC GRAPHICS'.