Learn to Crochet - Joining in New Yarn
When joining in new yarn or changing color, continue in the working yarn until two loops of the last stitch remain in the working yarn or color.
Yarn over with the new color or yarn and pull the new color or yarn through to complete the stitch.
Continue to work the following stitches in the new color or yarn following the pattern instructions.
If you are working whole rows in different colors, make the change during the last stitch in the previous row, so the new color for the next row is ready to work the turning chain.
If you are working a narrow stripe pattern, instead of cutting off the old color or yarn, carry it loosely along the side of the fabric so that it is ready to pick up again the next time it is needed. For wide stripe patterns, it is usually best to cut off the old color or yarn, leaving a 6" end for weaving in. Longer carries, also called "floats", are easily snagged.