Sometimes mistakes in color knitting, like any other kind of knitting mistake, warrant ripping out the work and trying again. But if you've only messed up your color a little bit, say, by working a few stitches out of order or in the wrong color, you probably don't want to have to rip back a bunch of knitting in order to fix them.
Some color knitting mistakes can be left and no one other than you and perhaps some other knitters will ever notice them. Then there are mistakes that are both small and obvious, like the mistake I made when I was practicing double knitting.
I'd gotten into the groove (yes, that is possible with double knitting) and wasn't paying that much attention to where my colors needed to change, such that I ended up knitting a square with a bite taken out of the edge.
There was no way I was going to rip back to fix it, but it did present me with an awesome opportunity to show you how to cover up mistakes in color knitting with duplicate stitch.
All you have to do is get a length of yarn of the color you need the stitches to be. Thread it onto a yarn needle and come up through the bottom of the V of the first stitch that needs fixing. Take the needle behind the stitch (as shown in the top part of the picture above), then back down to where you started in the center of the stitch.
Tug on the yarn so that it covers up the original stitch as best you can. In the second picture above you can still see a bit of the green behind, but that would be the case anyway because this is duplicate stitch.
Once you've worked one stitch, continue in the same manner across all the stitches that need to be covered. If yours aren't all right next to each other like mine, you'll need to use your judgement as to whether you can carry the yarn across the back of your work or if you need to cut the yarn and start again in the new position.