FileMaker Advanced 126.96.36.199, MacOS Sierra 10.12.5, MacBook Pro
Converting a field to a button by selecting it in layout mode and attaching a script using the Format > Button Setup menu item prevents you from being able to change the fill attributes.
How to replicate
In layout mode, click on an edit box field. Select Format > Button Setup and attach a script to it. In inspector, the objec type will change from "Edit Box" to "Group". Then try to change the fill on the field. It is locked, and the fill cannot be changed. It also prevents you from selecting the "Hover", "Pressed", and "In Focus" states in the Inspector to change the attributes of those states. It still has those states, and in browse mode will obey whatever formatting has been specified for them previously, but will not allow you to access them to change them so long as a script is attached to the field object with "Button Setup".
Workaround (if any)
Make a note of any script parameter on the field. Change the button definition to "do nothing". Change the fill attributes and states you need to change. Then reassign the button and reenter the previous script parameter.
I feel that defining a field as a button should not prevent you from being able to change its visual formatting or the attributes of its hover, pressed, or focused states.