Use layer colors option is grey

[Ben3B]

Hello I'm working on a file from a colleague

I click on the Use layer colors button , but nothing appends.

 

So I open an empty file , copy and paste stuff from the other file, and the Use layer colors works...

 

So go back in the colleague file , document preferences 
And the use layer colors (and black and white option too) option are grey .

I suppose that an option is checked somewhere, but I can't find it ... 
 

any idea ?

thanks