Neutral value
The neutral value for these is RGB = 128,128,128.
Background color
This is the solid color that will be used behind the geometry. It can be thought of as kind of the sky color.
Setting the horizon color in Blender will set this background color.
Back color - Ambient color
BK in psx term, it's equivalent to Blender's ambient color.
Lower values will result in stronger shadows.
Far color - Fog color
FC can be thought of as the PSX's fog color. It can be set in the engine's include/defines.h file for now, and will be the same for every level.