Carrying Colors On The Back - Color Change
Colorwork - Carrying Yarn Across The Back
Carrying your yarn across the back of your project is perfect if you only have 1 to 3 stitches of work in a row to change colors on. If you more than this, then Intarsia is a better option.
Carrying your yarn across the back also adds a bit of bulk, so again it's great for little bits of color, but not huge chunks of color. You can see in this blanket below that every other stitch has a new color while white remains the same, since it's only one stitch at a time that I need to carry my yarn past, it's quite practical to use this method.