You can change the colours by double clicking on them in the legend. Double click on a disk to drill down and find out what caused the change. This window is useful to get an idea of free space trends. You can spin the areas so free space is at the bottom which gives you a better idea of when you will run out. It also acts as a record of disk size upgrades.
Lost space is due to cluster allocation.
A file that is only 1K may still be allocated 32K of space. Directory Sizer records directory sizes on actual file length - not allocated space. Lost space is greatest on volumes with many small files and on large FAT volumes which have high cluster sizes by design.