There is no way to set the field shading to a different color--at least not
using Word's built-in settings.
However, you could use Find & Replace to change the font color/shading/etc.
I can't think of an easy way to automate it. But, I would probably create a
new character style (named Field, perhaps), to which I would apply the
desired formatting. I would then use Find & Replace to apply that style to
all fields in the document, which can be done in this way (turn field
shading off, or set it to "When selected", otherwise any shading you apply
via your style will not be shown; this is not necessary if you're changing
the font color, and not the shading):
1. Press Alt+F9 to toggle field display on.
2. Press Ctrl+H, and put ^d into the Find what: field.
3. Put ^& into the Replace with: field, and click Format - Style (lower left
corner of the Replace dialog), tap the first letter of the style (F, in my
case), click on the style, and then click OK.
4. Click Replace All.
5. Press Alt+F9 to toggle display off.
Why use a style? Because it then gives you an additional way to find styles
without having to press Alt+F9 and search for ^d. Also, if you change you
mind about how to display fields, you can do it by modifying the style, and
not resorting to find/replace.
Good luck.
