So I originally wanted to write a post with some advice for those wanting to tackle colorwork (e.g. fair isle) knitting for the first time. Once I uploaded all the pictures for this one, though, I decided it would be way too big for one post if I put up everything so stay tuned; there's more to come tomorrow.
Today's topic: what to do if you realize that you messed up on the row before (or even a couple of rows before). This is basically how it works with any kind of knitting. Frog not; just drop the offending stitch right off your needles.
Take your right hand needle (or an extra needle) and pull on the yarn to one side of the offending stitch, thus pulling it out of the loop (stitch) below.
... and here it is once you'e pulled the stitch out. Repeat as necessary for as many rows as you need. I prefer to fix one stitch at a time, though, for simplicity's sake.
Now stick your right/spare needle through that recently liberated loop (in this case grey) from right to left/front to back (things can get a bit shifty but hopefully you get the idea. If you pick this stitch up twisted or backwards it will show. Of course, it's your sweater I'm not gonna make you fix it. Then again, if you aren't gonna fix it right why fix it at all?).
Now that you've got the grey stitch on your right needle, what color should be above it? In this case white (tan/beige/whatever) is next. Note that this example was staged and the stitch was fine to begin with. Ironically when I picked this up and started knitting today, though, I'd done 3 stitches in a row wrong. Would that I'd gotten pictures of the real thing but alas this will have to do for now.
Anyway, find the beige strand (called a float) from the correct row and stick your needle under it. It should be behind the stitch on your needle. I think it's worth mentioning if it's in front of that stitch you'll end up with a purl not a knit stitch. This is kind of a biggie. Especially if you're fixing several stitches in a row or going down several rows. Picking up the wrong strands can mess up your tension and just generally ruin your day.
Now it's time to get your left needle involved. Stick it through the front of the grey stitch with the beige strand still laying across the right hand needle. It will basically look like you're about to do a normal knit stitch except that you're not hanging onto any yarn tails.
Now pull that beige strand through the grey stitch with the right hand needle (again just like you're doing a normal knit stitch)
It is fixed.
This is the same general idea you can use to fix a myriad of mistakes: a miscrossed cable, a knit instead of a purl, I can't think of anything else off the top of my head but for realz if you don't know how to do this it's worth learning.
Obviously this post is already very photo heavy so I'll be back with more colorwork tips tomorrow.