Inside Macintosh: Macintosh Human Interface Guidelines / Part 2 - The Interface Elements: Chapter 8 - Icons

Black-and-White Icons

You should begin by designing a black-and-white icon. In general, you should use an outline of one black pixel to create the icon border. Use a minimal number of black pixels in the icon so that the icon's appearance is noticeably different when selected. The Finder automatically inverts the black pixels
and the white pixels when the user selects the icon to create the selected appearance. Figure 8-19 shows an example of a well-designed icon that changes significantly when selected.

Figure 8-19 A well-designed icon and its selected version

If you use too much black or 50 percent gray in your icon, the icon
doesn't appear significantly different when the pixels are reversed for selection. Figure 8-20 shows an example of an icon with too much black
and 50 percent gray.

Figure 8-20 A poorly designed icon and its selected version