Interestingly, the red and the blue versions of these stickers are not just different colors, they are different products of different quality, each of them horrible in its own unique way.
Red ones are printed on thin glossy film. Light reflection makes it hard to see the characters under the stickers. And if your keyboard is black, you'll have a hard time seeing the characters ON the stickers, too. They are also very unforgiving to mistakes made during their application. You cannot unstick them: even if you just started applying a sticker and saw that something is wrong - can't remove it, it will be, most likely, ruined.
Blue ones are printed on thicker matte film. They might be alright, but, for some reason, they are covered with thick whitish glue, which is different from the one used in red stickers. Blue keys become grayish, and, again, it is hard to see the characters under the stickers, now because of that glue. The blue characters look good on black, and you can reapply these stickers. I might even start to like them (despite of a minor error on key ' 2 ' - it should have ' " ', not ' , ' on it). But that horrible glue started creeping from under the edges and sticking to my fingers, which I totally cannot tolerate.
As a result, I ordered vinyl stickers and finally found peace. I advise everyone who is willing to hear to do the same. Buy these stickers only if you absolutely must, like, you have allergies to vinyl, or you like it when your fingers stick to the keyboard, or you are looking for a present for someone you don't like.
WARNING: 
