Depends if you’re knitting flat or in the round. If you’re knitting flat and you have ONE row (or an odd number) without a color, then you need to carry it across, crossing over the yarns in back of the work every 3 sts so you don’t end up with long, loopy floats. If it’s 2 rows (or an even number), then just leave it at the side and it’ll be there waiting for you when you need it. If it’s more than 2 rows, be sure to catch and cross over the yarn every couple of rows, again, so you don’t get these long loops just sitting there.
In the round, odd or even doesn’t matter, because you’ll get right back to it when you come around to the beginning. But again, if it’s more than 2 rows, catch the yarn at the beginning of every other round or so, to prevent the loops.
In the case of a sparsely used color-- if it comes up regularly in the pattern, then carry it along in the back. If you go for many rows with out it, you may want to cut it and reattach it when necessary. But that’s between ROWS, not repeats in the pattern. For the pattern repeats, carry it along. The one exception to this would be if it only comes up in 1 place-- then I would do it by intarsia, that is, attaching it and leaving it just in that section.