Welcome to Gaia! ::

I suppose it depends on where in the pattern you switch the yarn, but my favorite way is using the Russian Join (here is another example). To me this is one of the easiest and seamless ways to switch colors in a project, but YMMV. I don't like having extra ends to weave in, so it's my preferred method. I also use it when just adding in more yarn of the same color.

Some folks just like to grab the new color and start knitting (dropping the previous color or cutting it off and leaving a tail to weave in). This works good if you make sure you weave the ends in REALLY well when you're done or you are in for some serious trouble heh. Some folks spit splice or felt (for those fibers that will do that).

On KnittingHelp.com (about halfway down the page) are a few videos with different ideas on how to change colors. I guess it's a matter of trying to find what works best for you.

Depends. Are you doing stripes, or a pattern? If it's a pattern you'll want to learn stranded colorwork: here's a youtube video of how it's done. If it's stripes and they're not too wide you can just carry them up.