Invisible seam

Hi, I’m a beginner - just started knitting a couple of weeks ago…I started making an afghan with all these individual sections that are supposed to be sewn together invisibly according to the pattern. How do I do that? There’s three different colors of yarn!
Love this site - it’s been such a big help - I LOVE the videos! It makes understanding how to knit so much easier than the book I got. :smiley:

Here’s a really great online tutorial on the mattress stitch:

With this type of seaming, yarn colour does not matter…the seam is invisible.