Can I set color fields by hex value
If you have created a Color Field the color selector dialogue uses your browsers built-in color selector.

On Chrome and Firefox, you should see a dialogue like this: 

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If you click over to the 2nd tab (indicated by the sliders) you'll be able to insert a hex value for the color in the input field at the bottom.

In Firefox you'll need to select RGB Sliders from the dropdown, then you'll see the Hex color input at the bottom.