I tried doing a tween from -80 up to 0 and it looked fine, brightening gradually. I went back in to the keyframe on frame 1 and dragged the control down, and the drawing became darker gradually until I reached -100, at which point it suddenly became slightly brighter and remained at that level of brightness no matter how low I set the brightness parameters. I can’t seem to get it to go completely black. At other times, I was able to set it to completely dark in the first frame, then bright in the last frame, but when I tweened it, instead of brightening gradually, it remained completely dark for several frames, then jumped to a brighter state in the middle of the animation and from there continued to brighten gradually. Is this some kind of open GL issue?
It really depends on what else and which other effects are also being applied.
Have a look at the function graph for the brightness values to make sure that the values are not hitting some strange numbers in between the defined keyframe values.