When you are knitting multi coloured tuck stitch patterns you only have one yarn threaded through the carriage at any time.Granny Hils said:
You have to change the yarn according to the tuck stitch pattern you are knitting, so that you knit a number of rows (usually an even number or the colour changes get complicated) and then change to the next colour.
This sounds more complicated than it is, but because changing the yarns manually slows the knitting process down quite a lot, people who often like to knit multi coloured tuck stitch or slip stitch patterns find it much easier and quicker if they have a colour changer. This allows up to 4 yarns to be threaded up through the colour changer, so that the yarns can easily be changed automatically when you take the carriage into the colour changer.