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.
You’ll then have a ready-made swatch based on the colour you chose in the step above. All you have to do is then sample the squares you like the look of with the Eyedropper tool.
I’ve kept this article pretty brief as I’m assuming prior knowledge of Photoshop. If anyone wants any help, leave a comment and I’ll respond or add to the article.