There are some important steps to know before working with video.
1. You need a codec on your system which can be used by Reaper. Best and easiest way is to install VLC 2.. or ffmpeg
https://www.reaper.fm/lgpl/ffmpeg-reaperdist.zip (just put it (unzipped) in your user plugin folder (Options-> Show Reaper resource path ...))
If Reaper is able to use them you can edit several video file types with ease.
2. If you load your video clips (per drag and drop from media explorer) you have to understand that overlapping videos result by default in only showing the video on top of the other track.
3. You can load an video processor as an FX into the media item or track to change color, size, position etc. of your clips. Also cutting, timing and speed are editable in the arrange window.
4. To export your video go to file -> render and choose an output format as your desired video format. Its super easy, because there are many presets to choose from. Under video ffmpeg are: avi, webm, flv, mp4 etc. (HD, Full HD, XGA etc.)
Please watch the tutorial videos i posted. They describe very well how to edit videos in Reaper. I was pretty astound how easy and stable it works and now its a new hobby. So give it a try and ask if you get stuck. Step by step!
Reaper community will help.
Edit: I forgot to mention, that if you got a green screen its a sign that you rendered an arrange area with no video item as a video file. Reaper was not able to render video. To make sure everything works fine on your system, follow the above steps (especially the codec thing) import a short video clip and an audio file. Put them on tracks and make a selection of the area where both play at the same time. Go to file->render-> choose: bounds -> time selection -> choose: output format -> video ffmpeg -> choose: format ... and hit render. If it works, you can start! Good luck!
