I don't know what this texture is called in the english version of the game but in swedish it's "FabriksFantastens tegelfasad med träkant". It is a brick texture with wooden corners. When using the dark grey version of this texture, the corners will get the light grey version and the other way around when using the light grey.
Are you up to date with all of the patches? Those textures were indeed swapped but were corrected with the latest patch. If you are already up to date, please try clearing your cache and repairing the game.
I just bought the game a couple of days ago and I am qute sure I checked the box for automatic updates. The version I have is 126.96.36.199.20. Is this the most recent one? I can try clearing my cache (if I can figure out how) but since I haven't been playing the game for more than two days and I had this problem from start, I don't see how it will help...
Not sure what the current borkedness status is, but if updating won't fix it for you here's a mod that solves this issue: http://www.modthesims.info/download.php?t=546278
July 2015 - last edited July 2015
That is the current version, @ElinLinaK. And yes, if you just bought the game, I can't see clearing your cache either. I'm at a loss though, as the textures are correct in my game. They were wrong prior to the last patch, but that patched fixed them for me. I will check with some other builders to see if they are correct or still glitched in their games...
Edit: after checking in with people on the forums, it appears that indeed, this glitch is fixed for some people but not for others, even though we're all patched to the same version. I don't have an explanation, but I'm adding it back to our list of active glitches on the Build Mode Feedback thread.