The color of a shaded rectangle cannot be recovered in some cases

Question

In the following MCE, a tcolorbox is equipped with two rectangles drawn as overlays. The color of these rectangles is supposed to be the same as the bottom color of the tcolorbox.

It works well with the color e.g. blue but not with cyan: why?

\documentclass{article}
\usepackage{tcolorbox}

\tcbuselibrary{skins}

\NewTColorBox{mybox}{ !O{blue} }
{
  enhanced,
  height=.75in,
  size=minimal,
  interior style={
    top color=black,
    bottom color=#1
  },
  overlay={
    \fill[#1]
    (frame.south west) rectangle +( .5in,-5)
    (frame.south east) rectangle +(-.5in,-5);
  }
}

\begin{document}

\begin{mybox}
\end{mybox}

\vspace*{5cm}

\begin{mybox}[cyan]
\end{mybox}

\end{document}

Answer 1

Apparently the color cyan is defined as CMYK color. However, to generate the gradient, the color is converted into RGB mode (see also this answer about gradients and CMYK mode). The conversion to RGB mode results in visible differences for some colors, which is especially the case for cyan.

You can solve this problem by telling the xcolor package explicitly to only use RGB mode:

\documentclass{article}
\usepackage[rgb]{xcolor}
\usepackage{tcolorbox}

\tcbuselibrary{skins}

\NewTColorBox{mybox}{ !O{blue} }
{
  enhanced,
  height=.75in,
  size=minimal,
  interior style={
    top color=black,
    bottom color=#1
  },
  overlay={
    \fill[#1]
    (frame.south west) rectangle +( .5in,-5)
    (frame.south east) rectangle +(-.5in,-5);
  }
}

\begin{document}

\begin{mybox}
\end{mybox}

\vspace*{5cm}

\begin{mybox}[cyan]
\end{mybox}

\end{document}