This is a handy technique if you are knitting a flat piece of knitting which changes colour after each row.
For my chunky scarf I used a circular knitting needles to knit the single row stripes without having to cut the yarn, and I would like to share this method with you. This is the only method I know of that allows you to work flat in single row stripes using only one ball in each colour. The bonus is that you have no yarn tails to weave in at the end of each row where the colour changes, plus you only need to have two balls of yarn (not a cumbersome set of bobbins).!
To do this you will need a circular knitting needle, but you will be working back and forth, not in the round.
Cast on your required number of stitches in colour A (black) onto a circular needle. Do not join in the round, you will be working flat (back and forth).
Row 1: in colour A (black), purl all stitches (wrong side)
Row 2: slide the stitches down to the other end of the needle, with the wrong side facing you. The working yarn (colour A, black) should be at the end furthest from the tip of the needle. Now attach the colour B (grey) yarn, and purl all the stitches. Both the working yarns (A and B) should now be at the same end of the knitting.
Row 3: in colour A, knit all the stitches (right side)
Row 4: slide the stitches down to the other end of the needle, with the right side facing you. The colour A working yarn should be at the end furthest from the tip of the needle. The colour B yarn should be at the tip of the needle. Using colour B, knit all the stitches. Both the working yarns (colours A and B) should be at the same end of the knitting again.
Repeat rows 1 to 4 to the required length.