> As I mentioned, it's simple to just repeat the use
> of the same 8 faces, for 9-16, 17-24, etc.

This is a very good idea.  Now this is implemented in isearch.

> I've added a third screenshot, where I did this:
> recycle the faces of groups 1-5 for groups 9-13.

Yes, recycling is nice, this is what now is used in isearch.

> The foreground for each group is apparently taken
> from the foreground of face `isearch'.  I guess I
> hadn't paid attention to that.  I think that's
> unfortunate, in general.  I probably didn't notice
> it because in my own setup I've customized face
> `isearch' to use a black foreground (and a green
> background).
>
> I think the default foreground for face `isearch'
> should be something that works well with each of
> the default group faces, and that's not likely to
> be `lightskyblue1'.  Change it to `black' and the
> regexp group highlighting is quite good.  You need
> to be able to see the text that the regexp parts
> apply to.
>
> I also tried just removing a foreground spec from
> the vanilla default for face `isearch'.  In the
> case shown, which is *scratch*, the text has face
> `font-lock-comment-face'.  And if face `isearch'
> doesn't have a good foreground spec (e.g. `black')
> then the result isn't great.  (But at least it's
> better than with foreground `lightskyblue1'.)
>
> As I've just noticed this problem now, and as I
> don't want to impose any default change on face
> `isearch' in my code, I've now changed the default
> regexp group faces to each specify a `black'
> foreground (instead of no foreground), for a light
> background mode.

You see?  So many problems when you tried to design
a color palette for many isearch faces.

I suppose you agree that the previous color palette in isearch
with a color gradient is not suitable for highlighting of group matches
because while looking at highlighted matches it's quite impossible to
say which color corresponds to which group number.
I meant when colors looks like a heatmap, for example: