It would be nice to have the option to pick the color which I want to set transparent in the video texture directly from the imported texture. It is a bit complicated to get the color if it is not plain black.
You’re right - it’s a bit complicated now, but I am afraid we won’t be able to add such a picker in the near future.
For now, I think it’s best to use an external image editor: paste a video frame into the editor, and use the editor’s color picker to identify the key color.
If your image editor uses sRGB (0-255) color space, you can use this online calculator: http://davengrace.com/dave/cspace/ to convert an sRGB color to a color in linear color space used by Shapespark. The calculator outputs the linear color in the 3rd row.
Thank you for the link @wojtek . This will make it a bit easier.