It depends on how the file you are working on was created. If the source file does not have grouped obects, this tip won’t help. Click the file in Inkscape and look at the bottom info to see if it says group of 2, 3, 4 etc. objects. If so, click ungroup, then click each section an change the color of that part of the file so you can address it separately in the Glowforge interface.
If you are working on this particular file, right click and save as svg. The file will be zipped (at least it was for me). Click the file frame.svg.zip and you should see frame.svg. Open frame.svg in Inkscape.