Support RGB and gray color models in ConTeXt

The `RGB` support was copied from the the Plain TeX definitions,
`tex/generic/pgf/utilities/pgfutil-plain.def:29-37`, which is from pgf-tikz#772.

The `gray` color model now maps to the grayscale and not to `rgb`.

Signed-off-by: Max Chernoff <49086429+gucci-on-fleek@users.noreply.github.com>

doc/generic/pgf/CHANGELOG.md

@@ -10,6 +10,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 ### Added
 
+- Add `RGB` and `gray` color model support for ConTeXt #1130
+
 ### Fixed
 
 - Typo in animations `end on` key #1273
@@ -37,6 +39,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 - Rocky Zhang (@rockyzhz)
 - Yukai Chou (@muzimuzhi)
 - Alexander Grahn
+- Max Chernoff
 
 ## [3.1.10] - 2023-01-13 Henri Menke
 

tex/generic/pgf/utilities/pgfutil-context.def

@@ -26,8 +26,17 @@
 \def\pgfutil@definecolor#1#2#3{\csname pgfutil@emu@#2\endcsname{#1}#3\@nil}
 
 \def\pgfutil@emu@rgb#1#2,#3,#4\@nil{\expandafter\def\csname\string\color@#1\endcsname{\xcolor@{}{}{rgb}{#2,#3,#4}}}
-\def\pgfutil@emu@gray#1#2\@nil{\expandafter\def\csname\string\color@#1\endcsname{\xcolor@{}{}{rgb}{#2,#2,#2}}}
+\def\pgfutil@emu@gray#1#2\@nil{\expandafter\def\csname\string\color@#1\endcsname{\xcolor@{}{}{gray}{#2}}}
 \def\pgfutil@emu@cmyk#1#2,#3,#4,#5\@nil{\expandafter\def\csname\string\color@#1\endcsname{\xcolor@{}{}{cmyk}{#2,#3,#4,#5}}}
+\def\pgfutil@emu@RGB#1#2,#3,#4\@nil{%
+    \begingroup
+    \pgfmathdivide@{#2}{255}\let\pgfutil@emu@RGB@r\pgfmathresult
+    \pgfmathdivide@{#3}{255}\let\pgfutil@emu@RGB@g\pgfmathresult
+    \pgfmathdivide@{#4}{255}\let\pgfutil@emu@RGB@b\pgfmathresult
+    \edef\pgf@marshal{\def\expandafter\noexpand\csname\string\color@#1\endcsname{%
+        \noexpand\xcolor@{}{}{rgb}{\pgfutil@emu@RGB@r,\pgfutil@emu@RGB@g,\pgfutil@emu@RGB@b}}}%
+    \expandafter\endgroup\pgf@marshal
+}
 
 
 % no need for x colors (users can load it if needed)