Some rows are only worked in one colour, but the next row is worked in both colours or just the contrast colour, so the yarn not being used must be carried across the back of the row BEHIND the stitches.
The following photos show how to carry the red yarn behind the whole of a white purl row.
Keeping both yarns at the back of the work, purl until a red stitch is reached, take red yarn across the top of the white yarn:
A red stitch is used as this conceals the carried yarn better, otherwise it might show through on the front of the work.
Bring white yarn to front and cross red yarn over the white yarn as shown below:
Work the stitch as normal using the white yarn, the red yarn is caught up by the white yarn and carried in a loop behind the work:
Continue purling for 3-5 stitches finishing on a red stitch:
The same method is used when working knit rows, by laying the colour being carried over the yarn being used to work the row. Always keep yarn being carried BEHIND the work.