Voxels can be either fixed or variable, and each of these can come with or without an additional custom colour. In BURRTOOLS all of these cases have their own specific representations in the 2-D grid as well as in the 3-D viewport. Figure Representation shows an overview of these possibilities. In this picture the default colour is red (= shape S3) and the custom colour is yellow (RGB = 1, 1, 0).
Fixed voxels always fill the cell completely in the 2-D grid as well as in the 3-D grid. In all the pictures of Figure Representation the voxels on the left are fixed voxels. Variable voxels only partially fill the cell in 2-D and have a black inset in 3-D (the voxels on the right in Figure Representation). For spheres the insed is simply a part of the surface.
Voxels that have a custom colour added (the yellow voxels in Figure Representation) show this colour as an inset in the 2-D grid, whereas in the 3-D viewer they are completely painted with this colour (provided that the Colour 3D View on the status line is checked, otherwise they will be painted in the default colour). Note that in both grids the default colours also have a slightly checkered pattern which can assist navigating in space (except for the spheres, which have no checkering).