How to start moss stitch temperature blanket (chain or chainless)

Hi! I am trying to figure out how to start my moss stitch temperature blanket. For each row, I am using two colors that represent the high temperature and the low temperature of the day. I am planning to change colors about halfway through the row. Because of this, I can't start with the first row of the blanket being the first day. I need to have some sort of starting row. I am planning to do a black border around the entire blanket, so I am wondering if it would be better to do a foundation chain in the border color and a row of single crochet in that color before switching to moss stitch for day one, or if it would be better to do a chainless foundation, and then to start moss stitch. I have a feeling that that would be the better option, but I don't know how to do the transition from the foundation to moss stitch. I am wondering if there's someone who can break that down for me. I want my blanket to be 300 stitches wide, but I'm not sure how many stitches that means I need in my foundation or if there's a turning chain. I'm also not sure how to do the color change from the border color to the day one color.