Changing thread or yarn colors in crocheting is really quite easy, actually. It's easier to change colors in crocheting then it is with knitting.
If you want to introduce another color, make a slipknot in your new color you want to bring into your work.
Photo belong etsy.com/shop/baboom
Pull it through the loop that is on the hook, keeping the knot at the back, pulling this down. You're ready to start working your three chains for your first stitch because we're actually working in trebles, and then working a treble into each of the next stitches. This is a very secure way of introducing a new color into your work.
For changing colors at the beginning of a row, using a half double crochet stitch, again complete all the stitches of the previous row, with Color A, except for the last stitch. With Color A work that last stitch, until you have three loops of Color A on your hook. Pick up Color B, again leaving a good length for your yarn end. Grab Color B with your hook, and pull it through the three loops of Color A that are on your hook. Chain 2 stitches with Color B, and turn. Now you're ready to do proceed in half-double crochet across your next row with Color B.
To keep the continuity of the item, make sure that the skeins are the same yarn weight. It is best to use the same brand of yarn through a whole piece.
As an example crochet hat beret
from the site
Composition - various yarns in purples: cotton, wool, mohair blends.