If you press cntrl + k you can change the background colour of the composition there.
Changing the background color of a comp will not change the rendered background color when using most formats. This is because the alpha channel is still transparent and alpha channels will render as black in all media players. The only reason to ever change the background color of a composition is to render specialized graphics where you need to pre-multiply the alpha edge with a specific color. That is an extremely rare project requirement. You should just keep the background color black.
The correct and only reliable way to have a background color is to fill it with pixels. An image, a solid, a shape layer, or footage will always work. If you turn on the transparency grid and you can see it, you will end up with black 99% of the time.
If you render with the alpha channel included then it will appear black regardless of the colour you've changed it too yes but if you're working on a composition that has no alpha channel then you can succesfully change the background colour of the composition this way and it will render that colour.
I've done this countless times instead of creating a solid layer as the background. I've also changed the comp bg colour when I've wanted to better see where the alpha might be showing, as changing it to a bright colour (for example, Magenta) can be easier to see than the transparency grid sometimes. So they're are reasons you may want to change it. More than one way to skin a cat and all that, depends on the project.
Jonathan_Lamb_06, The background color of the comp will only render with a few codes. It's a bad idea. I don't know any production houses that would follow that workflow. Simple comp with a red shape layer and the comp background set to green:
Send it to the media encoder and render using the industry-standard h.264 settings:
The comp background color switches to black because there are no pixels. There are only a few codecs and settings that will change the Black (0) alpha channel pixels to a pre-multiplied with black color.
The only way to get the background color of a comp to render is to choose a codec that supports RGBPre-multiplied color, which is the default in the Render Cue but there are many that think it should not be because almost all editing and compositing apps are set to use alpha channels as RGB Straight because you get better results and edges that are easier to control when you do composites. There is no option for RGB Pre-multiplied in the Adobe Media Encoder. Everything renders with a Straight alpha and that's why the comp background color turns black.
I know this, you're right. I just happen to use that feature when I want to view my text quickly on a background I can see it on and when my workflow allows me to render it. I was just trying to offer the OP some help. Just put a solid as your background, that's fine, job done.
If you press cntrl Y for a new solid layer, select a colour that contrasts with your text (in this case white) and then put that layer below all your other layers. This will ensure that it will render out the background colour as well. Hope this helps.
How do you put the solid layer under all of the other layers? And will that solid color show up in adobe media encoder? Thanks
Create a new solid Ctrl/Cmnd + y. Select the layer and drag it to the bottom of the timeline.
You really need to spend some time with the User Guide, I don't know anybody that has ever been successful using After Effects without doing some homework.
I just want to say, you are a horrible helper. One, there was really no reason to use this thread as an explainer for "alpha" or "specialised graphics." If you knew the answer, all that was required was "Ctrl/Cmnd + y, then drag that to the bottom."
Second, telling people to "do their homework" is borderline insulting, especially to a newcomer on AE. Was the question about getting good at AE? No, it was a simple question, and all that was required from you was a simple answer.
Really, I've seen your answers crop up all over AE threads, and all of them are complete utter jargon that is 99% irrelevant to the topic, given that beginners are the ones googling the questions. Just give short and sweet answers and leave it at that.