Why can I dye the glider but not the backpack?

They may be treated as separate items for customization. A dye option on the glider doesn’t necessarily mean the backpack has dyeable areas too. If the backpack has no dye channels in its appearance settings, its colors are probably fixed. The exact reason for the difference isn’t clear from the item behavior alone.

Check the backpack on its own in the appearance preview, rather than assuming the matching glider controls both. If there are no color slots there, it sounds like a limitation of that piece, not a setting you’ve missed.