ménageant des réserves où d’autres sont venus se loger

10:51 Jul 12, 2018 

"Ménager" is to use sparingly, to be economical, save (ex: space), set aside, provide (ex: a passage)

"Réserves" are portions/spaces left blank, any protected parts, open areas

"Se loger" is to find room, nestle, arrange.
Here, I would say "added" as suggested by Tony, or "incorporated", "contained", "supplemented".

I think "sont venus" shouldn't be translated literally. Like Tony, I would use "have been".

06:07 Jul 12, 2018 

I can't make any sensible suggestion as to exactly how to word it, but FWIW, here's my interpretation of what it means:

Successive over-painting means that some areas have been covered with white paint, leaving spaces where yet more colours have been added.

Effectively, applying patches white paint is a bit like making "holes" going back to the blank canvas behind, and then starting again!

Yes, looking at the pictures, you can see there's a lot of white, but this is not because the artist was too lazy to cover all the canvas!