Similar to the Callysto nbgitpuller bookmarklet, here's the code for a bookmarklet to open a Jupyter notebook from GitHub in Google Colab. Drag the following link to your bookmark bar/menu:
Click that bookmarklet when you are viewing a Jupyter notebook on GitHub, it will pop up a window with a link. Copy that link and you can use it to open the notebook in Colab.
If you prefer a bookmarklet that just opens the notebook in Colab (rather than giving you a link to paste somewhere) then use this one:
As always, comment if you encounter any issues or have any suggestions.