Grayscale images are alpha-blended, you see the average of two volumes, which means a darker image will darken the rendered output (unless you choose “Add” compositing option). Labelmap images are rendered as an opaque or semi-transparent overlay and so “background” region will not affect the rendered output.
Make sure you to click “labelmap” in the “Add data” window when you load labelmap/binary image (or if you already loaded as scalar volume, convert it to labelmap in Volumes module / Volume information section).
