This will probably work with other photo/imaging applications, but Photoshop is what I have used to create this template.
The file is basically just the same square smart object repeated, with varying transparencies over a white oblong and a black oblong.
All you need to do is edit the smart object (called 100%.psd), choose your base colour (make sure it is a strong colour so that the file has enough range of colours to play with), and save it again so that it updates the original swatch.psd file.
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