I'm very, very new to AE. So I purchased and downloaded this template- http://www.wavebreakmedia.com/stock-photo-video-footage-p-193304/Opening-Doors--AE-Version -4.html
the background and the images of the door aren't showing up. What do those funky colors mean? Is it that those files are missing or something?
Yes, the color bars mean that the source item that that layer depends on is missing. Usually, with template projects, the idea is that you import your own footage and replace it in some specified layers.
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