Using any random color-picker online to go from RBG<->Hex works plenty fine, but, I think CLI tools are cool. I'm still pretty new to the Linux scene, and I really enjoy finding random command line programs to play with. I searched around a bit on Github and the AUR and couldn't find any CLI color conversion utilities, which I honestly found surprising (maybe I didn't look hard enough).
I decided I'd really enjoy trying to make a simple command line program for RGB<->Hex, and the scope slowly expanded. I could see myself expanding it (maybe including more color formats, suggesting complementary colors) but for now, unless people discover ways to break it or offer any pull requests, I think I'll be moving on to other projects.
It has no dependencies besides Python, and can be easily installed on mac/linux (see the README for more). I'm not certain how you'd set it up as a command line program on Windows, but you can just use Python in cmd to use the script anyway!