coltitle in subtitle not working.

[Tetragramm]

The subtitle always inherits the color of the main title, instead of being overridden by specific options.
Small example:

\begin{tcolorbox}[colback=red!20,colframe=red,size=small, boxrule=0pt,
    subtitle style={coltitle=black, boxrule=0pt, toprule=1pt, bottomrule=1pt, colback=white},
    title={First Title}]
    First Content
    \tcbsubtitle[coltitle=black]{SubTitle}
    Second Content
\end{tcolorbox}

produces

Similarly, setting the main coltitle to black produces

[muzimuzhi]

\tcbsubtitle is itself a tcolorbox in which the subtitle is actually the upper part of the box, hence you need colupper=black instead of coltitle=black both in subtitle style={<options>} and \tcbsubtitle[<options>]{<subtitle>}.

Not sure if it's too late to add redirection for title options used in <options> of \tcbsubtitle[<options>]{<subtitle>}.

[T-F-S]

I think, @muzimuzhi has given all needed explanations. So, using colupper instead of coltitle for the subtitle does the trick.

\documentclass{article}
\usepackage{tcolorbox}

\tcbuselibrary{skins}

\begin{document}

\begin{tcolorbox}[colback=red!20,colframe=red,size=small, boxrule=0pt,
    subtitle style={boxrule=0pt, toprule=1pt, bottomrule=1pt, colback=white},
    title={First Title}]
    First Content
    \tcbsubtitle[colupper=black]{SubTitle}
    Second Content
\end{tcolorbox}

\begin{tcolorbox}[colback=red!20,colframe=red,size=small, boxrule=0pt,
    subtitle style={colupper=black, boxrule=0pt, toprule=1pt, bottomrule=1pt, colback=white},
    title={First Title}]
    First Content
    \tcbsubtitle{SubTitle}
    Second Content
\end{tcolorbox}

\end{document}

[Tetragramm]

Sorry for disappearing. Perhaps add a clarification to the documentation? Page 26, where subtitle is described talks about how it's an independent tcolorbox, which implies the same settings work. One of the examples could be given a different title color, or maybe a list of settings that don't get inherited (if there's more than one).

[T-F-S]

Sorry for disappearing. Perhaps add a clarification to the documentation? Page 26, where subtitle is described talks about how it's an independent tcolorbox, which implies the same settings work. One of the examples could be given a different title color, or maybe a list of settings that don't get inherited (if there's more than one).

The next version will have an example with redish subtitles:

[T-F-S]

Documented in https://github.com/T-F-S/tcolorbox/releases/tag/v6.0.0