even though on Premiere Pro it has been edited into black and white.
And how? It seems you are either experiencing some DynamicLink issue where your color corrections do not render or you fundamentally misunderstand something. Technically even a b/w video is just RGB data since aside from some old Quicktime CoDecs there are no genuine greyscale formats. AE wouldn't handle anything else, anyway. Well, whatever, your post is too vague and you have not provided any solid technical information, so it's nigh on impossible to tell what you are actually doing and what goes wrong. You need to provide more info.
I am not understanding your workflow from the explanation you have given. After Effects is designed to work on shots and occasionally short sequences. There is very little reason to bring a whole film into After Effects unless you want to do some color correction to the whole film that could not be better achieved in some other way. The alpha channel has nothing at all to do with the color channels. Turning it off will not change color footage to black and white.
Many of the effects in Premiere Pro will not come into AE when you use Dynamic Link or import your Premiere Pro project as a comp. If you want to work just on the silent film portion, which is supposed to be black and white you can put that portion in a separate sequence in Premiere Pro, then bring the entire silent era section into AE or render it to a production format DI and do your effects work.
As Mylenium said all video is RGB or RGB + Alpha. That's how video works. If there is an alpha channel it only defines areas of transparency. That's all it does. If you were use one of the channel effects to modify or disable the alpha channel you would only remove transparency if it existed and fill it with black or you would make the entire image transparent so none of the RGB pixels were visible.
I don't understand what you are trying to do or why you are bringing the footage into AE but from your description it sounds like you don't understand color channels or video. Please explain what you are trying to do so we can point you to an efficient way of getting it done.
I added the black and white effect on Premiere Pro to the silent film part of my short film. The problem is that it's not showing in black and white. I'm wanting to use AfterEffects to add a damage plugin to my video.
You have two choices. Turn the film black-and-white in After Effects and add film damage or render the black-and-white section from Premier then apply film damage and AE. Almost all effects you add in Premier will not come into AE using dynamic link because they are not compatible. The only thing you can do about that is render your footage or do all of the effects work in AE.
This is a workflow problem not a bug.
Thanks for your help. It's annoying that the Premiere Pro effects aren't compatible with After Effects.
Not surprising though because the rendering pipeline is completely different. Adobe is working on a better way of rendering for both apps that is more efficient but there is still a very long way to go. In the mean time thing of workflow like this. Trim and edit in Premiere Pro, if you need AE, use it when you need it, final edit in Premiere Pro, rough cut audio in Premiere, final audio mix in Audition, then final color grade. It's different tools but the same workflow that has been used to make movies since the first scene was cut and the first visual effects were produced more than 100 years ago. You write, shoot, pick the shots, process the shots you want, add the sound trim them to make an effective story, mix the sound and then do the final color grading before you print. Movies longer than a couple of minutes have never been made in a single pass. In fact, until non linear editors movies were always cut in 10 minute reels because 10 minutes is 1000' of 35mm film and that's about all that will fit on a reel.
When I cut any show longer than about 10 minutes I break each scene into separate sequences and then create a final sequence using all of the other sequences as clips. it makes it so much easier to go back and make changes to the story. It's the way most professional editors work on features. Cut the scenes, arrange the scenes into acts, tie the acts together and then send the final cut off for sound mixing and color grading.