1. No. Dye color is an indicator, but not a determining factor. Variations of good dye can be several colors. And inversely, variations of bad dye can be several colors. Worse yet, some plastic platters are dyed, to make the disc dye appear false. (Example: Memorex colors discs.). Now realize this refers to CD-R only; all DVD-R/+R should be purple, and all DVD-RW/+RW should be gun-metal gray.
2. Myth. DVD-R is slightly more compatible than DVD+R (~5%), but other than that, there are no real differences. In fact, they all contains the same basic ingredients: polycarbonate upper and lower, dye, silver/alloy (sometimes gold) foils. The only major difference is in how the data is written to the disc, and even that's mostly the same. The old DVD+R Alliance used to publish "DVD+R better than DVD-R" that never happened,
3. The class difference are not really for longevity. Once a disc has burned, they should all last about equal: anywhere from a few decades to a century. The difference between the classes is the quality of burns, either as coasters (failed burns) or absolute burn quality. The latter one affects how it plays and reads in players/ The only exception is Ritek dye, pre-Fuji oxonol, as that one has proven to fail sooner. How much sooner is debatable. (Hint: It's not 2-5 years, as boogeyman stories would suggest.)
4. When written. Technically, it's both, but at a MUCH slower rate unburned (almost undetectable)
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5. When are you? (I peaked at the IP, Brazil. Is that correct?) Most of the better media have to be bought online these days, as only cheapo junk is sold in stores. The one exception being computers stores like (in the USA) Fry's, Microcenter and maybe Tigerdirect stores.
Amazon.com,
Newegg.com, Meritline.com and Supermediastore.com have the good online.