I understand you've got a solution with the contraption, but if you do want to make your own .hex file, essentially the .hex files are indeed just normal text files, where each line is the hex code for a colour in the palette. So you can just make them in a text editor (e.g. notepad) and save with a .hex extention.
In order to get the hex codes, Google Search actually has a built in colour picker at https://g.co/kgs/iDpdLzf which may be handy for this.