To fix the purple checkerboard issue with several Portal textures in Hammer:
1) Download this file: dload.php?action=file&file_id=4
2) Unpack it in to your C:\Program Files\Steam\SteamApps\Username\Half-life 2 Deathmatch\hl2mp
3) Cancel your eye appointment.
Note: I did not create/make this, So please do not give me any credit for this. I got permission from the person who made this to post this.
And have fun no more black and puprle textures xD
Also, the creater wanted me to post his SteamID page.
