Lots and lots of iOS apps allow the input of custom colours, so it's too bad Marvin does not.
It's not a question of EITHER - OR. I would keep the current list of colours (categorising it into submenus so the scrolling to find a particular colour is faster), and the final item at the bottom of the list could be "Custom...". Tapping it would open the colour picker and/or the option to input the exact RGB/hexadecimal code.
For more, see
https://github.com/kguil/Marvin-Roadmap/issues/126