You can use
To match the example given, the following expression gets the trailing space background color:
(face-attribute 'whitespace-trailing :background)
face-attribute is a compiled Lisp function in `faces.el'.
(face-attribute FACE ATTRIBUTE &optional FRAME INHERIT)
:around advice: `ad-Advice-face-attribute'
Return the value of FACE's ATTRIBUTE on FRAME.
If the optional argument FRAME is given, report on face FACE in that frame.
If FRAME is t, report on the defaults for face FACE (for new frames).
If FRAME is omitted or nil, use the selected frame.
If INHERIT is nil, only attributes directly defined by FACE are considered,
so the return value may be
unspecified, or a relative value.
If INHERIT is non-nil, FACE's definition of ATTRIBUTE is merged with the
faces specified by its
:inherit attribute; however the return value
may still be
unspecified or relative.
If INHERIT is a face or a list of faces, then the result is further merged
with that face (or faces), until it becomes specified and absolute.
To ensure that the return value is always specified and absolute, use a
default for INHERIT; this will resolve any unspecified or
relative values by merging with the
default face (which is always