Adobe Illustrator how do I make font fill "color" be inverse of what is beneath it

You can save the inverted photo as a pattern by dragging the image into the swatches panel

Then use it as a fill for the text 

I'm not exactly sure what you are asking, so if I'm misinterpreting your question, I apologize.

What I'm understanding is that when your type is over a dark background, you want the type to be white. When the type is over a light background you want the type to be black. 

Illustrator will not do this automatically. You have to select the type (as shown in your screenshot) an apply a Swatch color from the Swatches panel.

Hope this helps!

John,

One way is to place the inverse image precisely above the original, as you already have, and the text/Type at the top in the stacking order (expanded Layers panel), the select the text/live Type and the inverse image and hold Ctrl/Cmd and press 7 to turn them into a clipping set.

The Clipping Mask (the text which can be live Type) will be the Clipping Mask showing the part of the inverse image immediately beneath and hide the rest of it, so that the original image outside the text/Type and the inverse image inside fit precisely.

Edit: Hi Susan.

Edit edit: I just realized we used reverse ways.

Embed a copy of your photo in your document, placed directly above the existing one. Use Edit > Edit Colors > Invert Colors on the embedded copy of your photo. Postion your text in back above the inverted image. Select text and inverted image. Use Make Mask (with Clip and Invert Mask Checked).