> > (read-from-minibuffer (concat "What " (propertize "do you want" 'face
> >                                                   'bold)))
> 
> So, more than one face, one each for every part of the prompt?

Maybe.  Depends on what you need/want.

It's not the minibuffer, but see attached screenshots of
isearch prompts (from `isearch+.el').

The screenshots with the white background show the default
faces.  The screenshot with the light orange background shows
the face values I myself use.

There are these faces used for parts of the Isearch+ prompt:

1. Standard prompt face (`minibuffer-prompt')
2. Regexp search face (`isearchp-regexp')
3. Wrapped search face (`isearchp-wrapped')
4. Overwrapped search (`isearchp-overwrapped')
5. Multi-buffer search (`isearchp-multi')

In addition, the `*wrapped' faces are used in the mode-line
lighter.

(In the screenshot that shows the mode-line lighter, the
lighter text is `R*SEARCH', which indicates case-insensitive
regexp search.)

Wrt the minibuffer prompt (not Isearch), here is an example:

(message
  "You can invoke command `%s' using `%s'"
  (propertize (symbol-name cmd) 'face 'icicle-msg-emphasis)
  (propertize (key-description binding) 'face 'icicle-msg-emphasis))

A screenshot shows this too, but using my setup (where red
is the normal prompt color for my standalone minibuffer).