It all depends, there are club bands, like the NPA, AU, ADA, NBRC, these are seamless. Then there are split and coil bands. Seamless bands must be put on, while the bird is still very young, and stays on the bird until it dies. Think of it as a "social security card" for your birds. For example, I have a Birmingham roller, with a leg band of "NBRC 07 1800." That means the band is from the national birmingham roller club, the "07" is the year he was born, and the 1800 is the ID number of the band. If you plan on showing your birds, you will need Seamless bands on them, unless the show allows for the birds to have a split band. As far as color, many clubs choose the color of the band for the year. But split bands are usually up to you, or what the supplier has on hand. Split band can be switched from bird to bird, and are good for marking hens from cocks(if you wish), or as tempoary ID's for your birds.
Peace
Chris