On 19.12.2018 0:35, Juri Linkov wrote:

> I tried to replicate your settings: in emacs -Q I change the default
> foreground with e.g. M-x set-foreground-color RET

That only seems to change the default's foreground color. Which affects 
how the context looks, but not lines with changes.

> A side note: M-x set-foreground-color RET TAB shows a list
> of colors with different backgrounds, not foregrounds
> as it would be natural to expect, so it is difficult to decide
> what foreground color to select by looking at background colors.
> This needs a separate bug#.

Indeed, it only gives a rough idea.

> Ok, let's say I chose M-x set-foreground-color RET LightSeaGreen
> 
> Now in diff-mode I see exactly the same fontification with the same
> foreground colors and as seen in the original buffer visited from
> diff-mode by `C-c C-c', plus red/green background added by diff-mode.

Let me show you a screenshot. I've set some purple-ish foreground using 
'M-x set-foreground-color', and you can see it in certain places. But 
where there was the "default" face in the original buffers, in the 
changes lines there is now green4 or red4 in the foreground, coming from 
the respective faces diff-added and diff-removed.

(You can also see the white from the docstring and comment faces, but 
that's a smaller issue, and I don't mind changing the theme if it comes 
to that).