In viewer mode, only green channel is shown in histogram, red and blue channel are missing. The histogram in the info pane of brower is OK.
It's a new bug introduced in 0.56. The former 0.51 is all right.
0.56 Linux x64: only green channel in histogram
Moderators: helmut, XnTriq, xnview, Dreamer
Re: 0.56 Linux x64: only green channel in histogram
... Right, by default, in full screen too ... but you have just to do a right mouse click and choose RGB instead (or RGB+Luminance) 

XnViewMP Linux X64 - Debian - X64
Re: 0.56 Linux x64: only green channel in histogram
You are right! Thank you. 
