Box half filled color

Welcome to TeX.SE. From the next time try adding a compilable MWE, so that it will attract more users to answer your question.

You can start with (borders):

\documentclass{standalone}
\usepackage{tikz}
\begin{document}
    \begin{tikzpicture}
    \filldraw [black] (3, 2) --(3,0)--(0,0) plot [smooth, tension=2] coordinates { (0,0) (1,1) (2, 1) (3, 2)};
    \filldraw [red] (3, 2) --(0,2)--(0,0) plot [smooth, tension=2] coordinates { (0,0) (1,1) (2, 1) (3, 2)};
    \draw[yellow,  thick] (0,0)--(0,2)--(3,2)--(3,0)--cycle;
    \end{tikzpicture}
\end{document}

to get:

and I leave the rest of customisation for you to do ;).

If you prefer tcolorbox:

\documentclass{article} 
\usepackage[most]{tcolorbox}

\begin{document}
\begin{tcolorbox}[
    sharp corners, boxsep=5mm, boxrule=.5mm,
    colback=brown, width=5cm, 
    halign upper=left, valign upper=top,
    halign lower=right, valign lower=bottom,
    enhanced, segmentation empty,
    underlay = {
        \begin{tcbclipinterior}
            \filldraw[fill=orange, draw=tcbcolframe, line width=.5mm] (frame.south west)--++(90:1cm) to [out=5, in=185] ([yshift=-1cm]frame.north east)|-cycle;
        \end{tcbclipinterior}
}]
    A
    \tcblower
    B
\end{tcolorbox}
\end{document}