Not a stupid question! If you already have a hex file you can just drag it from your file folders onto your Decker projects and it should change your colors automatically.
If you're on some platform where you can't drag-and-drop you can copy-paste the PalImport contraption (scroll down a little bit in this post to find the code block to copy) which should let you find and open your hex file and then apply it with the button.
Alternatively you can mess around with the colors directly in Decker with Missooni's RGB adjuster (expand the second part of this post and then copy the code block)
There's also an art section in Phinxel's Field Notes which talks about how color works in Decker, in case seeing things explained in a different way is helpful.