How to combine fonts with luaotfload?

Question

Consider this MWE:

\documentclass{article}
\usepackage{fontspec}

\directlua{luaotfload.add_fallback("noto_fallback", 
{
    "NotoSerif:mode=base;color=FF8800;",
})}

\setmainfont{Source Serif Pro}
[RawFeature={fallback=noto_fallback;color=A0A0A0}]
\setsansfont{Source Sans Pro}

\begin{document}
\section*{Some Polytonic Greek}
\setlength\parindent{0pt}
\setlength\parskip{1.5ex}

Οὐχὶ ταὐτὰ παρίσταταί μοι γιγνώσκειν, ὦ ἄνδρες ᾿Αθηναῖοι,
ὅταν τ᾿ εἰς τὰ πράγματα ἀποβλέψω καὶ ὅταν πρὸς τοὺς
λόγους οὓς ἀκούω· τοὺς μὲν γὰρ λόγους περὶ τοῦ
τιμωρήσασθαι Φίλιππον ὁρῶ γιγνομένους, τὰ δὲ πράγματ᾿
εἰς τοῦτο προήκοντα,  ὥσθ᾿ ὅπως μὴ πεισόμεθ᾿ αὐτοὶ
πρότερον κακῶς σκέψασθαι δέον.

\sffamily
As you can see, \emph{Source Serif Pro} (grey) contains some, but not 
all required glyphs, which are then pulled from my fallback 
font \emph{Noto Serif} (orange). I'd like all Greek characters to be 
taken from Noto Serif (i.e., be orange in this example.)

\end{document}

My text is mainly in English (German, actually, but this doesn't matter), but requires some interspersed Polytonic Greek. I have a font which could handle all my Greek needs specified as a fallback: How can I get LuaLaTeX to pull all Greek glyphs from that font, for consistency's sake?

Answer 1

babel package can detect which Unicode block a glyph belongs to and automatically change fonts, but punctuation and digits (and space) are in the Latin quarter of Unicode and so don't change even when they are surrounded by Greek.

MWE

\documentclass{article}
\usepackage[greek,english]{babel}
\babelprovide[import, onchar = fonts ids]{greek}
\babelfont[greek]{rm}[Colour=FF8800]{Noto Serif}
\setmainfont{Source Serif Pro}
\begin{document}

 Some text here. Consider the following:

Οὐχὶ ταὐτὰ παρίσταταί μοι γιγνώσκειν, ὦ ἄνδρες ᾿Αθηναῖοι,
ὅταν τ᾿ εἰς τὰ πράγματα ἀποβλέψω καὶ ὅταν πρὸς τοὺς
λόγους οὓς ἀκούω· τοὺς μὲν γὰρ λόγους περὶ τοῦ
τιμωρήσασθαι Φίλιππον ὁρῶ γιγνομένους, τὰ δὲ πράγματ᾿
εἰς τοῦτο προήκοντα,  ὥσθ᾿ ὅπως μὴ πεισόμεθ᾿ αὐτοὶ
πρότερον κακῶς σκέψασθαι δέον.


\end{document}

polyglossia handles text and punctuation (and other things) but requires manual switching with \selectlanguage{greek}, so not automatic.

MWE

\documentclass{article}
\usepackage{polyglossia}
 \setmainlanguage{english}
 \setotherlanguage[variant=polytonic]{greek}
 \newfontfamily\greekfont{Gentium Plus}[Colour=FF8800]

 \begin{document}

 Some text here. Consider the following:

\selectlanguage{greek}
Οὐχὶ ταὐτὰ παρίσταταί μοι γιγνώσκειν, ὦ ἄνδρες ᾿Αθηναῖοι,
ὅταν τ᾿ εἰς τὰ πράγματα ἀποβλέψω καὶ ὅταν πρὸς τοὺς
λόγους οὓς ἀκούω· τοὺς μὲν γὰρ λόγους περὶ τοῦ
τιμωρήσασθαι Φίλιππον ὁρῶ γιγνομένους, τὰ δὲ πράγματ᾿
εἰς τοῦτο προήκοντα,  ὥσθ᾿ ὅπως μὴ πεισόμεθ᾿ αὐτοὶ
πρότερον κακῶς σκέψασθαι δέον.

\end{document}

Answer 2

In most cases it is best to use the babel support Cicada mentioned since it can also adjust e.g. hyphenation patterns and has a nicer user interface, but if you only care about the font then luaotfload has a multiscript feature which works similar to fallback, except that it selects the font based on the used script instead of using the first which has the glyph. This also avoids the issue with punctuation babel seems to have:

\documentclass{article}
\usepackage{fontspec}

\directlua{luaotfload.add_multiscript("noto_greek", 
{
    % Could be: grek = "NotoSerif:mode=base;color=FF8800", as in your example, but especially for Polytonic Greek the language and script should always be set and `mode=base` is except for math fonts almost always a bad choice.
    grek = "NotoSerif:mode=node;color=FF8800;script=grek;language=PGR",
})}

\setmainfont{Source Serif Pro}
[RawFeature={multiscript=noto_greek;color=A0A0A0}]
\setsansfont{Source Sans Pro}

\begin{document}
\section*{Some Polytonic Greek}
\setlength\parindent{0pt}
\setlength\parskip{1.5ex}

Οὐχὶ ταὐτὰ παρίσταταί μοι γιγνώσκειν, ὦ ἄνδρες ᾿Αθηναῖοι,
ὅταν τ᾿ εἰς τὰ πράγματα ἀποβλέψω καὶ ὅταν πρὸς τοὺς
λόγους οὓς ἀκούω· τοὺς μὲν γὰρ λόγους περὶ τοῦ
τιμωρήσασθαι Φίλιππον ὁρῶ γιγνομένους, τὰ δὲ πράγματ᾿
εἰς τοῦτο προήκοντα,  ὥσθ᾿ ὅπως μὴ πεισόμεθ᾿ αὐτοὶ
πρότερον κακῶς σκέψασθαι δέον.

\sffamily
As you can see, \emph{Source Serif Pro} (grey) contains some, but not 
all required glyphs, which are then pulled from my fallback 
font \emph{Noto Serif} (orange). I'd like all Greek characters to be 
taken from Noto Serif (i.e., be orange in this example.)

\end{document}