Most websites you find online will say that DVD-R is the most widely compatible format, although to be perfectly honest, I've never had any compatibility problems with the 200+ DVD+R discs I've used...

Dual-layer basically means the disc has two layers with which to store/burn information onto, giving it almost twice the storage capacity (from 4.5GB to about 8.7GB), but you do need a DVD-drive that can actually support them to be able to use both layers.

Not really sure why there are so many variations of colours, but I've always used silver/titanium, and to my memory, I've not had to throw a disc away yet.