\documentclass[11pt]{article}
\usepackage[margin=1in]{geometry}
\usepackage{callout-itembox}
\usepackage{parskip}
\pagestyle{empty}

\newcommand{\sectionrule}[1]{\bigskip{\bfseries\Large #1}\par\smallskip}

\begin{document}

\begin{center}\bfseries\LARGE callout-itembox --- colour gallery\end{center}
\medskip

% ===================================================================
\sectionrule{Built-in colour themes}
Every theme sets the bar, background and heading colour together.
Call it with \texttt{theme=<name>}.

\callout[theme=blue]{theme=blue}{Deep navy bar on a light blue-grey
background. This is the default if no theme is given.}

\callout[theme=red]{theme=red}{Strong red bar on a soft peach background.}

\callout[theme=teal]{theme=teal}{Dark teal bar on a pale teal background.}

\callout[theme=green]{theme=green}{Forest green bar on a light green
background.}

\callout[theme=purple]{theme=purple}{Deep purple bar on a faint lavender
background.}

\callout[theme=orange]{theme=orange}{Burnt orange bar on a warm cream
background.}

\callout[theme=slate]{theme=slate}{Blue-grey slate bar on a cool neutral
background --- a quiet, low-contrast option.}

% ===================================================================
\sectionrule{Same themes, body only (no heading)}
\callout*[theme=blue]{Body-only blue.}
\callout*[theme=red]{Body-only red.}
\callout*[theme=green]{Body-only green.}
\callout*[theme=orange]{Body-only orange.}

% ===================================================================
\sectionrule{Numbered markers}
\callout[theme=blue, number=1]{First}{Use \texttt{number=1} for a numeral
instead of the dot.}
\callout[theme=purple, number=2]{Second}{Each theme keeps its colours.}
\callout*[theme=slate, number=A]{Alphabet, body only.}
\callout[theme=green, number=1, vertical]{Vertical number}{Add
\texttt{vertical} to rotate the numeral up the bar.}
\callout[theme=purple, number=12, vertical]{Multi-digit}{Works for any
number, e.g. \texttt{number=12, vertical}.}
\callout[theme=teal, number=ABC, vertical]{Multi-alphabet}{Works for combination of alphabets, e.g. \texttt{number=ABC, vertical}.}

% ===================================================================
\sectionrule{Rounded corners}
\callout[theme=teal, corners=rounded]{corners=rounded}{The colored bar
follows the curve.}
\callout*[theme=red, corners=rounded, arc=8pt]{Custom radius with
\texttt{arc=8pt}, body only.}

% ===================================================================
\sectionrule{Drop shadow}
\callout[theme=blue, shadow=on]{shadow=on}{A soft drop shadow lifts the box
off the page.}
\callout[theme=red, shadow=on, corners=rounded]{Shadow + rounded}{Combines
with rounded corners.}
\callout*[theme=green, shadow=on]{Shadowed, body only.}

% ===================================================================
\sectionrule{Custom one-off colours}
If no theme fits, override any of \texttt{barcolor}, \texttt{bgcolor},
\texttt{headcolor} with any \texttt{xcolor} expression.

\callout[barcolor=magenta!70!black, bgcolor=magenta!7,
         headcolor=magenta!70!black]{Custom mix}{Built from
\texttt{xcolor} percentages.}

\callout[theme=blue, barcolor=black]{Override just one}{Here only the bar is
changed; the background and heading still come from \texttt{theme=blue}.}

\definecolor{brand}{HTML}{6A1B9A}
\definecolor{brandbg}{HTML}{F3E9F7}
\callout[barcolor=brand, bgcolor=brandbg, headcolor=brand]{Defined colours}%
{Pass your own \texttt{\textbackslash definecolor} names for brand palettes.}

\end{document}