A few ideas:
1) Are you in 256 colors mode, or 256 shades of grey mode? The latter is optimized for greyscale monitors and, at least in System 6/7, would display the Apple Menu icon as solid rather than shades of grey. Confusingly, many color elements remain on screen when you use this greyscale mode on color monitors -- just not all.
2) Did your installation of 7.5.5 involve a network boot disk, disk tools, or other kind of "emergency" utilities disk in any way? These Systems often had color resources stripped out of them to save space.
1) Are you in 256 colors mode, or 256 shades of grey mode? The latter is optimized for greyscale monitors and, at least in System 6/7, would display the Apple Menu icon as solid rather than shades of grey. Confusingly, many color elements remain on screen when you use this greyscale mode on color monitors -- just not all.
2) Did your installation of 7.5.5 involve a network boot disk, disk tools, or other kind of "emergency" utilities disk in any way? These Systems often had color resources stripped out of them to save space.