I took a shot at improving the suggestions from the autocomplete hex feature initially. Though I think the only real solution here is to prevent the form from changing the text color value while you are typing.
We do still need to check the value to try and ensure we have a valid hex code, but I’ve changed the approach so that it only checks the value if you click or tab out of the field or when submitting the form.
Hopefully this change will work better for you:
https://github.com/discourse/discourse/commit/202493cbeca15ed1700fe51516531fea3575a367