If this is too complicated, can I just join a ndw color and start a new square?
Yes. you could do it that way.
It will depend on the main st patttern you are doing, but I've just finished one worked like that. the ridge between colours is created with a few rows garter stitch. (2 ridges)
You could do 2 rows garter st, change colour then 2 more rows of garter st before continuing in pattern.