I only bought my first glass insulator because it was a pretty aqua blue, and I thought it would make a neat paperweight. That was 10 years ago.
I still have the piece, and over the intervening years it's been joined by several others in varying shades of the greenish-blue color to which I'm partial. None of them are rare, and I doubt that I will ever become a hardcore collector willing to pay the hundreds or thousands of dollars that some insulators command. My interest was piqued, however, by their utilitarian origins.
Collectible insulators are objects of low conductivity used to support and isolate communication and power transmission lines http://www.insulators.com/general/parts.htm. The earliest date to 1844 when they were used on the telegraph line between Washington, D.C. and Baltimore, and over which Samuel Morse transmitted his first telegraph message http://www.nia.org/timeline/1840.htm.
Insulators are generally separated into three broad categories: glass, porcelain, and all other materials (rubber, plastic, wood, etc.). Within these categories, pieces can be arranged according to several different criteria including manufacturer, shape (or style), color, country of origin, etc. Due to the sheer number of variations generated over 160 years of manufacture, most collectors specialize in just one or two areas of the field.
Of course, whether you're a novice or a seasoned collector, knowledge means having no regrets, either for the piece you paid too much for or for "the one that got away." To that end, you will need reliable and accurate sources of information. One of the best on the Web is the Glass and Porcelain Insulators Reference Site at http://www.insulators.com. Here you'll find a glossary of terms, style number charts, online copies of manufacturers' catalogs, photos, and more. There are also numerous articles by members of ICON (Insulator Collectors On the Net) that cover everything from the pitfalls of collecting to how to clean train smoke from a newly acquired piece.
Another Web site that you absolutely should not miss is that of The National Insulator Association http://www.nia.org. The NIA represents thousands of collectors from around the world, and their site offers everything from a detailed history and identification of insulators to scanned images of vintage catalogs and advertisements, a list of regional clubs, and convention information. One of the most intriguing articles on the site concerns the artificial altering of insulator colors http://www.nia.org/altered/index.htm. This is extremely important information when you consider that some common aqua insulators generally worth $3 or $4 can be, through exposure to radiation, turned into $500+ sapphire-blue gems.
Speaking of color, a 1909 Electric Appliance Company catalog offered insulators in "red, blue, and amber glass" http://www.insulators.com/general/faq.htm#colorcost. Since no one has ever seen a genuine red glass insulator, beware: it's probably a fake.
It might make a neat paperweight, however.
The Definitive Guide to Colorful Insulators, by Michael Bruner
Price Guide for Insulators: A History and Guide to North American Glass Pintype Insulators (2003), by John and Carol McDougald