you can change yarn in any row. just if you do it in some places it will show.
whenever you see the "bumps" on the front - be it garder stitch or RS purl and WS knit - you will see the color change by a bi-colored stripe.
with RS purl and WS knit that is not avoidable (ribbing or pattern for example) but in garder stitch: alway change so that the bumps are on the WS of your piece.
as long as your front is stochinette: go ahead and change to your hearts desire.