Apple has certain guidelines for the creation of icons (of course, they do!). For more information (I haven't covered everything), check out the section on icons in Inside Mac VI. I don't know where this info is in the new Inside Mac editions.
• Design for black-and-white first, then add color.
This kinda follows the "never-tried-but-still-true" caveat for writing a term paper: Create an outline, then write the paper from the outline. Yeah, it's a great guideline, but I don't know anyone that follows it. Personally, I always create for color, but I keep the black-and-white version in mind as I do.
• Use light colors for large areas; use bright colors for details.
To avoid garish-looking icons, you don't want great swatches of hot pink. Keep things toned-down for the most part. The best examples of this are the icons for the standard System 7 control panels.
• Never try to create 3D effects in black-and-white icons.
Usually, they just end up looking sloppy. Keep this rule in mind if you create color icons first. Occasionally, if the effect is subtle enough, I'll use 3D (usually just a one-pixel drop shadow); but even in these cases, the 3D effect is far from the most important aspect.
• Small icons should reflect simplified large icons.
The small, 16x16-pixel icons (ics#, ics4, ics8) icons, used in Small Icon view and in the Apple menu, should contain all of the elements of their larger, 32x32-pixel counterparts (ICN#, icl4, icl8), but in a simpler, less detailed form.
• Use the 16 Apple standard colors for 8-bit icons.
Well, you don't HAVE to, but if you use lots of custom icons it'll keep things looking good. Use dithering to create new colors from these 16 (but then I don't 'cause I don't like dithering).
(Apple, by the way, loves to dither colors in 4-bit icons. Dithering is the process of checkerboarding two colors so they blend into one to create a new shade. This only works in large areas. Take a look at the 4-bit System 7 control panel icons.)
Chris Holmes has certain guidelines of his own:
• Don't use large black areas in black-and-white icons.
Sometimes you just have to, of course; for instance, my black floppy disk icon is one large black area. But for the most part, I tend to stick to pure outlines. Again, take a look at the (black-and-white) System 7 control panels.
• Pay attention to color depth.
The black-and-white icon is the most important; it's displayed when no other version of the icon is available. Second is the 8-bit (256-color) version; most people with color have at least 8-bit color now. If you're looking to save space, don't create a 4-bit (16-color) icon.
• Keep the style consistent.
I try to make my icons look as realistic as possible. Occasionally, I make a cartoonish one. Never mix the two concepts. It just looks goofy.
• Take advantage of icons in the system software.
If you need an icon of a Macintosh, copy it out of the System file. If you're designing a control panel, model it after existing Apple control panels. There's a great wealth of well-designed icons available in the system software (though some of it's well hidden in the individual control panels, extensions, desk accessories, etc.).
DO NOT directly copy existing icons if you intend to use your icons in a commericial capacity.
• ResEdit Tip: Cmd-click on a new color to change all pixels of the same color.
If you Command-click on a new color in the color palette, all pixels in your icon that matched the previously selected color will change to that new color. For example, if you want to change all the blue pixels to red, select blue in the color palette (or, to make sure you select the right blue, use the eye dropper tool and click on one of the blue pixels in your icon), then Command-click on red; all of the blue pixels will change to red.
