How to join two colors and knit them vertically

I am knitting a baby romper. Each leg is its own color. At the crotch, the pattern says to join yarn, but to continue knitting the body with two colors… essentially continuing the two colors from the legs into the body. Any tips on how to switch from one color to another in the middle of the row?

Just cross the yarns over each other in mid row. Otherwise you’ll get 2 separate pieces that aren’t connected to each other. This has to be a flat project, not in the round, because it won’t work in the round. Pretend the front of the romper is 40 sts across, 20 of each color, Red and Blue, in stockinette st. You knit 20 of red, cross the yarns over each other, and then making sure the blue is snuggly pulled around the red-- remember, you don’t want a hole or gap between the 2 yarns-- and then knit 20 blue. On the next row, purl 20 blue, cross the yarns over each other, again, making sure the red is snuggly pulled around the red, and then purl 20 red. Repeat these 2 rows. If you want, try doing it with 10 sts of each color on a swatch, to get the hang of it.

The technical name for this is Intarsia and you can find a video under the video tab and advanced techniques if you need more than Sandy gave you.

THANK YOU! Great instructions to solve my conundrum.