There isn’t a way to add a hex value for a color directly in Twenty Nineteen, however there’s a plugin called “Options for Twenty Nineteen” by Web LTD, which allows hex colors and other customizations.
(I’m not connected to the company/creator. I just discovered this because I had the same frustration. I’ve been using it on a website I’m building and it’s working really well for me. I think there’s a free version, but I ended up paying for it. The price is in…Euros? or Pounds?… and it came to $14.37)
