Document Classes ⇒ Passing rgb colour syntax

[Vaite]

I want to pass a colour scheme using the syntax

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[red]
[rgb][0.98,41.0,0.08]
[RGB][249,104,21]

Two optional arguments are used to pass the color specifications to this. But such implementation results in

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Package xcolor Error: Undefined color `RGB'.

Here are the details

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\documentclass[a4paper,12pt]{article}

\usepackage{xcolor}

\ExplSyntaxOn

\NewDocumentEnvironment{that}{O{orange}ommm}
  { 
    #3 #4 #5
    \color{#1}{#2}
  }
  {}

\NewDocumentEnvironment{this}{O{orange}omm}
  {
    \begin{that}[#1][#2]{Something}{#3}[#4]
  }
  {
     \end{that}
  }

\ExplSyntaxOff

\begin{document}

\begin{this}[red]{Name}{Surname}
  Some Text.
\end{this}

\begin{this}[RGB][249,104,21]{Name}{Surname}
  Some Text.
\end{this}

\end{document}

[Bartman]

You pass the name of a color model as the name of a color. Please read the description of the \color command.

One way to achieve what you want would be

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\documentclass[a4paper,12pt]{article}
\usepackage{xcolor}

\ExplSyntaxOn
\NewDocumentEnvironment{that}{O{orange}ommm}
  { 
    #3 #4 #5
    \IfValueTF{#2}% xparse on texdoc.org
      {\color[#1]{#2}}
      {\color{#1}}
  }
  {}

\NewDocumentEnvironment{this}{O{orange}omm}
  {
    \begin{that}[#1][#2]{Something}{#3}[#4]
  }
  {
    \end{that}
  }
\ExplSyntaxOff

\begin{document}
\begin{this}{Name}{Surname}
  Some Text.
\end{this}

\begin{this}[red]{Name}{Surname}
  Some Text.
\end{this}

\begin{this}[RGB][249,104,21]{Name}{Surname}
  Some Text.
\end{this}
\end{document}

I'd find it clearer and easier to name a color using the \definecolor command in the preamble.