Border color different, how to manage 2 colors

Hi… I would like to make a blanket with a 3 inch border in seed in a contrasting color for the body of the blanket.

I am new to knitting and do not know the best way to manage the yarn or the side border once I get the bottom 3 inches done.
Do you use separate balls of yarn for each side and pick them up when you get to the border? So would that mean taking a skein and creating 2 separate balls? How would you estimate how much yarn for each side?

I am thinking of doing a baby blanket … maybe the BigBadBabyBlanket where the border is a contrasting color to the main / center of the blanket.
Probably something about 30 inches in size of so.

Thanks for your help…

although i’m not as experienced as a lot of knitters here, having just finished that pattern myself, i would use two separate balls for each edge. you can create two by pulling from each end of the skein or using two separate skeins.