In content HLG H.265

Regarding editing HLG, Shotcut does not currently operate on HDR content. There is an option to convert to edit friendly with an advanced option to convert to 709 colorspace. If you convert to 709, you can edit the files, and the final output will look OK, but will not be HDR/HLG.

Regarding color grade, you can search specifically for that topic in this forum. Here are a few samples:

https://forum.shotcut.org/t/color-grading-tutorial/24744