Mattress stitch is my absolute favourite thing to teach because it is honestly like magic!  It gets me every time how clever it is!  You can use mattress stitch to sew up the edges of any knitting really, but it works best when you have two sections of stocking stitch (like you would on the side seam of a jumper).

In theory, because of it's super cleverness, you can use any colour yarn for mattress stitch.  I'd recommend using the yarn you knitted in, but if you are using a really thick yarn you might want to use something different,