I'd like to know more about color adjustments. I know GIMP can do them in some form, but I have no idea what the options are or what kind of results I should hope to be able to achieve. Documentation doesn't really seem to address these questions, though it is available if your question is "how do I run this menu item I found?" (Answer: open the menu, and click on the menu item.)
Cropping and resizing are trivial. It would take less than one minute to learn how.
You can use the gmic plugin alongside with Gimp, and adjust the CMYK values. I use that all the time. Color adjustments like yellow -> orange, or green -> purple, might exceed your expectations. Photoshop is excellent at it as well, i have never used Photoshop to compare though.
For more professional work Clut (Color Look Up Table) is supported by gmic as well.
I also made my own open source tool to pass gmic filters to whole videos, see an example here [1].
There's a Colors menu in Gimp 3. But it's destructive and only for the selected layer, not the special layer type you get in Photoshop.