I always had one idea how to have almost Source-like RGB decals in HL without any mods. Simply make 3 separate textures for each of the RGB channels and put all 3 decals into same spot. A bit expensy but u have fully RGB,
768 color ^^ decals.
Also I just found the working of env_sound very interesting. Believe it or not but it affects the player in a very similar manner explosions does in HL! Turns out you not only need to be in it's radius, but you also have to see the env_sound entity itself for it to affect you. This way it doesn't affect you through walls, even if you're inside the radius...
EDIT: I just also noticed that for the transition from one env_sound to another to be successful you have to stay in the new one for at least half a second or something. If the radius of the new env_sound is so small that by longjumping or gaussjumping you can pass through it in miliseconds, the new env_sound won't take effect. The solution is to simply set a bigger radius for the new env_sound so you stay longer in it. Thanks rufee for testing.