I recently discovered a new, very handy method of joining a new strand of yarn. Usually, I prefer the Russian join (splice the yarn ends, put them a bit through eachother, spit on it and roll between your hands – the ends will felt together), but that only works for pure wool, not for superwash or other materials.
The back join goes as follows: when you reach the end of your yarn (black), take the end and double it. Take the new yarn (red) and string it through the double end of the old yarn. Now double that too. Leave tails of about 8 centimeters (you can cut them off after the next row).
Now, knit the last part of the old yarn and the first part of the new yarn doubled. On the next row, knit just one stitch into each doubled stitch (you don’t want to increase, just knit them together). You can cut off the tails later (keep about 1-2 centimeters to avoid the ends going to the RS).
You can also use this join to start a newly introduced colour in a specific spot. For that, you will basically do the same technique, but you need to know where exactly to put the join for the new colour to show up in the right spot. Knit until where you want the new colour to start. Now, mark that spot on the old yarn (with a paperclip or something else, maybe even your finger). Tinker back three stitches. Now make the join, taking care to cross the yarns exactly on the spot that you indicated previously. Knit the three stitches again (this time with the yarn held doubled) and continue in the new colour (held doubled as well). If the join is not exactly in the right spot, adjust and do it again until it is right.