I believe that viewbuilder buttons has a different threshold for adjusting font color (between black and white) depending on the background. The button background color in the screenshot is the same, but the font is differently recolored. The same thing seems to be true for bitmap colors.
(Alternatively, it could be an inconsistency only when setting the viewbuilder button color manually. I haven’t experimented.)
PS. I’m using cLib to get the Renoise preferences (color settings), aiming to make tool colors more consistent with the Renoise GUI. That’s the reason for my nitpicking 
7041 !(https://files.renoise.com/forum/uploads/7041-color_inconsistency.png)
