Hi David! Thanks for your answer. That's a nice solution, but it doesn't work for me, as there are not only text styles, but also some non-GREPpable object styles.

It would be great if there is any possibility to make a color variable (like text ones) with the condition like “if chapter number/section prefix = X, then KEY COLOR = A”, but I've never heard of something like that in Indesign.

So I stay with my model of manual color replacement in each chapter, as for the moment it's the quickiest one. Hope we can find more solutions :)