Problem crochet-joining pieces

the first two are the two sides of the knitting I did. the third is what the pieces is supposed to look like at the end.

I’m making a hat, and it calls for a crochet joining method to hold together the pieces, but whenever I try to do it, it winds up making a little stripe because I’m using a new color for the joining.
What am I doing wrong? Is there some way I can make that little line dissapear so its just a raised edge?

If nothing else, I just want a detailed series of pictures or possibly a video detailing how to join two pieces of knit fabric via a crochet straight-stitch.Preferably with a depiction of both sides of the fabric once it is done.

Can you post what pattern it is? I have no clue but that might help people answer you if they can look at a picture of it.