Sry fraylocus but what do you mean with this phrase? "For example, with the inverted mode CoolReader indistinguishable letters and only the screen is displayed in black."
Inverted mode meaning with the original firmware? Can you re-post your phrase in spanish? I speak italian so maybe I'll understand better than your english.
EDIT
(I think you mean that in Coolreader the text is more black than grey, compared to the stock firmware. But I'm finding out that in epubs double tapping for dictionary lookup doesn't work at all)