Tartans and Clan colorsIf you have some family affiliation with a clan, you'll probably want to have a kilt made in the tartan pattern of that clan. For those of us without Scots ancestry, well, pick one you like.
Some people can be very sensitive about someone wearing a color which they aren't "entitled" to, but I haven't yet met anyone who is Scottish who is mortally offended by a non-member sporting the clan's colors. (However, I've found it is fairly important to at least know which tartan you are wearing, and even more helpful to know something about them when faced with a kilt-wearing Scotsman wanting to know just exactly who you are.)
When you are in doubt, or have not even the slightest imagined connection to a clan or a sept of a clan, there are also regional tartans. For example, the Black Watch and Royal Stewart are considered "free" setts and can be worn by anyone.
Colors and VariationsTartans for any particular clan usually have several variations. A Hunting sett may be a more muted version of the Dress.
There are usually several accepted modern versions of a clan's tartans:
Tartan SamplesThere are several collections of bitmapped or .GIF images of various clan tartans, if you want to get a feel for the patterns.