Hi Dave,
I don't know of a way to change the font color, but in those situations in our instance I've set a Section on the custom form and labeled it 'Reporting fields' with a note that explains that they can be ignored.
In use-cases where I needed calculations or fields that shouldn't be visible/updatable by everyone, I've also manipulated the sharing settings for that specific section. You could alter so that it's admin-only visible if the users truly don't need to see it at all for example, or make it visible to anyone but editable only by people who can Manage the form etc.
Katherine