This is the the one color interface currently.
The hex value should be #RRGGBB, which seems to match what you show here,
so maybe I'm not understanding the problem you're seeing when you say "the HEX # does not relate to RGB".
Also, if you have suggestions for other things you'd like to see in the color picker or elsewhere, feel free
to add them here: https://adobexd.uservoice.com/forums/353007-adobe-xd-feature-requests