CD-R Color Is A Guide To Longevity
The color of the reflective foil and the color of the dye determine the CD-R's final color. The foil usually doesn't play a major role in CD-R lifespan. Although aluminum foil can oxidize enough to affect recoding, this is rare and happens only in extreme conditions. Gold foil is almost totally inert, and hardly ever causes trouble.

The dye type matters more, because dyes have different chemical compositions, and thus different life spans. The four major dye types used in CD-Rs can be identified by their intrinsic color:

• Cyanine is the most widely used and cheapest CD-R dye. It looks bluish (as its name suggests) or blue-green when backed by a silver foil, but can look distinctly green when backed by gold foil.
• Estimates of the dye's useful life range from 10 years to as high as 75 years. But data-storage is an inherently conservative business, so it's wise to assume the worst and plan only on a lifespan of 10 years or so. This makes cyanine-based CD-Rs OK for most routine backups and legal archival purposes. But they may be less acceptable for very long-term historical, business, or family archives, unless you plan to re-record the data onto a new or different medium every decade or so.
• Phthalocyanine is an-almost transparent, very light aqua color, and usually is backed with gold foil, so a phthalocyanine-based CD-R usually looks golden. For this reason, and because the dye is exceptionally durable, it's sometimes called the "gold standard" of CD-Rs.
• Although phthalocyanine CD-Rs may be expensive overkill for routine storage, they're tops for long-term archiving, with a reputed shelf life of something like 100 years.
• Formazan is a light green color, and is usually backed by gold foil, producing a greenish gold CD-R. It's actually a hybrid of cyanine and phthalocyanine, and combines the qualities of both.
• Metallized AZO is a dark blue, and is normally used with silver foil, which gives the CD-Rs a dark blue appearance. Metallized AZO is said to have a long shelf life, approaching that of phthalocyanine.

Sometimes, Extremely Long Life Is Pointless
You might think that the longest shelf life is automatically the best choice, but that may not be so. For example, a CD-R with a 100-year shelf life sounds great, but who knows what technology will be like 100 years from now? Who knows what hardware will be able to read the data on the CD-Rs you produce today? Even now, just one human generation into the computer revolution, enormous quantities of old data stored on early tapes and large-format floppies are all but inaccessible. The data's there, but no one makes the hardware to read it any more!

So, the "sweet spot" in selecting a CD-R type for any given data storage task is the one that provides enough longevity to accomplish the storage task with a comfortable safety margin, but no so much as to needlessly raise the costs by paying for unnecessary longevity.

For example, most of my CD-Rs are daily backups that, if all goes well, I'll either never need to use, or will use only rarely. So, mechanical wear of the CD-R isn't an issue. Enormously long life also isn't an issue. Because operating systems and applications evolve over time, most system-restoration backup data loses value after a couple years and--vanity aside--little of my user-generated data has significant value after 7 to 10 years.

For these reasons, the simplest, cheapest CD-Rs are perfectly adequate for my routine backups, and I shop for these CD-Rs solely by price. Sometimes, a promotional deal on high-quality, name brand CD-Rs makes them cheapest; other times, it's the basic unprotected-foil CD-Rs that are cheapest. They all work fine in my CD-R burner, so I don't worry about the specs, and just let price be my guide for most purposes.

My one exception is when I back up selected, critical long-term business records, family photos, digital movies, and the like. Then, I'll use a midrange CD-R to help ensure the data will last as long as I need it to, which is to say, to the time when a better backup medium comes along. When that happens, I'll simply re-record onto the new medium. I've been doing that for years.

For example, I have some E-mail files dating back more than 15 years. I saved the original files on 5.25-inch floppies, and transferred the files to newer media over the years: from single-sided/single-density floppies to double sided/density; to 3.5-inch floppies; to tape; and now, to CD-R. At some point, these E-mail records--along with all the other data I want to keep for a very long time--will get transferred to writable DVDs, and then to whatever comes after that. This way, the data is always kept fresh, and isn't dependent on any single backup medium's longevity.