How to set the color for whitespace in isearch failures

Question

I have my isearch failure face set like so:

(custom-set-faces
 ...
 '(isearch-fail ((t (:background "#660c00"))))
 ...
)

It works for the most part, highlighting text which doesn't match in that color. However, whenever I write failing whitespace, i.e. a space or a tab which has no match in the buffer, the face that is used to highlight it is a brighter shade of red which is not the same color as the rest of the unmatched text. Furthermore, this color of whitespace is replaced by my custom isearch failure color when non-whitespace characters are written after it.

Here is an example demonstrating this.

I suspect that I can solve this issue by setting some face somewhere, but I'm not sure which face is the culprit here and I've looked and looked for it to no avail.

FYI, I'm using GNU Emacs 25.1.50.2 on OS X 10.11.6.

Answer 1

M-x list-faces-display is your friend.

The face used for this is face trailing-whitespace. It highlights, well, trailing whitespace. ;-) Customize it to whatever you want: M-x customize-face trailing-whitespace. You can even make it identical in appearance to face isearch-fail.

You can see how face trailing-whitespace is used in the code by looking at function isearch-message, in library isearch.el (use M-x find-library isearch).