EntityFX terrain,1
This is Full Bright, which means lighting is ignored.
The other problem, as Sunteam said, is most likely the blending.
Multiply mode is multiplying numbers between 0 and 1.
These get small very quickly. The Additive blend is thus
the dominant dominant factor.
With EntityFX set to 0 the ambient light will have an effect.
But after all the blending that effect is very small.