Hello, thanks for trying out the shader!
A white pixel shouldn’t be necessary, although both palettes should have the same width and height (transparent pixels included).
In your case, if both palettes have the white pixel at the end removed, the shader should still work.
If it isn’t working, please open an issue on the repository, so we can troubleshoot it better :)
This is the link to do so: https://github.com/nadjiel/eye-dropper/issues/new?template=bug_report.md