I'm just gonna put how to reproduce this here:

1. add a block to the editor and set it to a colour other than black (#FF0000 for bright red as an example)
2. Make a test animation with 2 frames (default interpolation selection)
3. Frame 1 shows the correct colour so leave it and go to frame 2
4. Switch Frame 2's colour to something else (#00FF00 as an example for green)
5. hit play
6. it cuts to black, fades in green, then fades back to black as if red wasn't used at all, even though red is the default colour for the block, and frame 1 still says that red is supposed to be used there, clicking it says it's red in the colour selector too.

Making the animation then adding the block does this too, so either order of steps 1 and 2 equal the same thing.
Happens to the colour filter in the effects tab too.