Lots of people have posted things about adding more colors or size to the icons. Good, but incomplete.
Icons - including database, workspace page, toolbar, and bookmarks -- should allow you to specify a standard windows (supported by linux and mac also after a fashion) icon (.ico) files. These files can be stored as a file resource within the database easily enough.
This way of storing icon data handles mutliple sizes which the ui can then select from for best use in a particular place.
Supported colors and resolutions span from very rough to almost photographic clarity, and many tools exist for creating and editing them.