Drawing a cone and coloring it without its basis

Question

How can I draw a cone and color it except its basis (circle). Then drawing an arrow from the colored part and adding a comment?

Answer 1

This is an attempt to create a somewhat realistic shading for the cone.

\documentclass[tikz,border=3mm]{standalone}
\usetikzlibrary{fadings}
\tikzfading[name=fade cone,
left color=transparent!100,
right color=transparent!100,
middle color=transparent!30,shading angle=100]
\begin{document}
\begin{tikzpicture}[upper cone/.style={insert path={
    (174:2 and 0.4) arc(174:6:2 and 0.4) -- (0,3) -- cycle}}]
 \draw[fill=blue,upper cone];
 \begin{scope}
  \clip[upper cone];
  \fill[white,path fading=fade cone] (-2,-0.4) rectangle (2.7,3);
 \end{scope}
 \draw (174:2 and 0.4) arc(174:366:2 and 0.4);
 \draw[stealth-] (0.3,1.4) -- ++ (2,1) node[right,align=left]{some\\text};
\end{tikzpicture}
\end{document}

One could make it more interesting by shading the inner side as well, and more versatile by dropping some of the hard coded values in favor of computed quantities.

\documentclass[tikz,border=3mm]{standalone}
\usetikzlibrary{fadings,calc}
\tikzfading[name=fade cone,
left color=transparent!100,
right color=transparent!100,
middle color=transparent!30,shading angle=100]
\tikzfading[name=inner cone,
left color=transparent!30,
right color=transparent!30,
middle color=transparent!100,shading angle=80]
\begin{document}
\begin{tikzpicture}[upper cone/.style={insert path={
    (174:2 and 0.4) arc(174:6:2 and 0.4) -- (0,3) -- cycle}}]
 \draw[fill=blue,upper cone];
 \begin{scope}[local bounding box=fore]
  \clip[upper cone];
  \fill[white,path fading=fade cone] 
   let \p1=($(fore.north east)-(fore.south west)$),
   \n1={-\x1*cos(100)/2},\n2={max(\x1,\y1)} in
  (fore.south west) rectangle ++ (\n2+\n1,\n2);
 \end{scope}
 \draw[fill=blue] circle[x radius=2cm,y radius=0.4cm];
 \begin{scope}[local bounding box=back]
  \clip circle[x radius=2cm,y radius=0.4cm];
  \fill[black,path fading=inner cone] 
   let \p1=($(back.north east)-(back.south west)$),
   \n1={-\x1*cos(100)/2},\n2={max(\x1,\y1)} in
  (back.north east) rectangle ++ (-\n2-\n1,-\n2);
 \end{scope}
 \draw[stealth-] (0.3,1.4) -- ++ (2,1) node[right,align=left]{some\\text};
\end{tikzpicture}
\end{document}

Of course, this is not really realistic.