Opacity for the pre-comp problems

It looks like there is only one layer in the main comp. If that is true, and you have changed the composition background color and are animating opacity, the Comps background color has nothing to do with the rendered color of the layer. You are, in effect, working with a black background because there are no pixels, and only a couple of codecs will interpret transparently with a forced background color. Did you follow that? 

In other words, set a composition background color to white, have transparency in the comp and render, and you will not get a white background because transparency in most codecs that do not support RGBA (Alpha Channels) is black. If you want to have the opacity effect, the rendered color of a layer, you must add a colored solid below the layer. That explains why there is no lightness change in your render. 

Actually it looks as you designed it. Based on your screenshot you are lowering the layer's opacity to around 75 percent and against a black background this will look like a darker purple. If you want full black you have to dial it down to zero, if you want it lighter you have to change the composition background in the comp settings to white or put a white solid underneath your other layer.

Mylenium 

Hi Doodledum1,

Thanks for writing in.

What do you mean by "light"? You have a black background. Lowering the opacity in this case, will darken the scene instead of making it light. Also, your minimum opacity value is 75% which could be tough to notice when you have a moon-rising shot in the beginning. You can try lowering the opacity value further if that's what you're going for.

Let us know if this is not what you're looking for.

Thanks,

Nishu