+ Is it a SCP addition? (Just to paint it in red).
Yes, added after 3.6.9 was released (ie, in late January). It will be an official feature starting with 3.6.10, in other words.
+ Which are the default values if it is not specified?
The default values are the ones that used to be hard-coded. Those being:
255, 127, 31 for the LARGE1, LARGE2, MEDIUM, and ASTEROID explosions
191, 191, 255 for the standard WARP effect
191, 255, 191 for the KNOSSOS WARP effect
+ RGB coded? GBR? BRG?
RGB, to keep it the same as all other tbl color specifications. There is no alpha component however, so it's only valid to have the 3 values for RGB.
And as stated earilier, values of 0, 0, 0 will give off no light at all when that particular effect occurs. If you wanted dark/black light you would have to make at least one of the RGB values "1", or simply use 1, 1, 1 for a uniform dark color.
+ I suppose tbm supports it.
Yep. And it's completely optional in both tbl and tbm.