That said, of course if you want to view color content on an EPD then it is quite necessary and not a gimmick at all.
When we went from black & white photography to color photography we also introduced a lot of tradeoffs. Would you have considered color photography a gimmick ?
It sucked or was massively inconvenient for the first 50+ years.
Biology spent a ton of time adapting and evolving for the ability to see color but sure, it's a gimmick. I guess you should consider syntax highlighting a gimmick in code editors as well? Charts/graphs in textbooks use color as a gimmick, any book that include illustrations/images in color are gimmicks