TIKZ: only fill 75% of the node-background with color

You can use path picture and path picture bounding box:

\documentclass[tikz,margin=5mm]{standalone}
\usetikzlibrary{calc}
\begin{document}
\tikz
\node[draw]
  [path picture={
    \fill[gray!30](path picture bounding box.north west)rectangle
        ($(path picture bounding box.north east)!.75!(path picture bounding box.south east)$);
  }](n)
  {Some Text}
;
\end{document}

This works also if the node is a circle:

or a cloud (options: cloud,cloud ignores aspect, needs library shapes.symbols):

Only if the node is a rectangle, you can use the node name instead path picture bounding box:

\documentclass[tikz,margin=5mm]{standalone}
\usetikzlibrary{calc}
\begin{document}
\tikz
\node[draw]
  [path picture={
    \fill[gray!30](n.north west)rectangle($(n.north east)!.75!(n.south east)$);
  }](n)
  {Some Text}
;
\end{document}

---

Update (because of a comment)

If the background should be colored in an intervall, eg. from 0.3 to 0.8, use

($(<name>.north west)!.3!(<name>.south west)$)
rectangle
($(<name>.north east)!.8!(<name>.south east)$);

Replace <name> by the node name or path picture bounding box.

Code:

\documentclass[tikz,margin=5mm]{standalone}
\usetikzlibrary{calc}
\begin{document}
\tikz
\node[draw]
  [path picture={
    \fill[gray!30]
      ($(path picture bounding box.north west)!.3!(path picture bounding box.south west)$)
      rectangle
      ($(path picture bounding box.north east)!.8!(path picture bounding box.south east)$);
  }](n)
  {Some Text}
;
\end{document}

Doing the fill in a separate step might be an option.

\documentclass[tikz,margin=5mm]{standalone}
\usetikzlibrary{calc,backgrounds,scopes}
\begin{document}
\begin{tikzpicture}
\node[draw] (a) {Some Text};
{ [on background layer]
\fill[gray!30] (a.north west) rectangle ($(a.south east)!0.3!(a.north east)$);
}
\end{tikzpicture}
\end{document}

I propose something similar: shading

\documentclass[tikz,margin=5mm]{standalone}
\begin{document}
\tikz\node[draw, top color=gray!30,bottom color=white]{Some Text};
\end{document}

EDIT

Since it was requested, here is a sample with a fading; I set the background as orange, in order to make the effect apparent

\documentclass{article}
\usepackage{pagecolor}
\usepackage{tikz}
\pagecolor{yellow!30!orange}
\usetikzlibrary{fadings} 
\begin{document}

\tikzfading[name=fade down,
top color=transparent!0,
bottom color=transparent!100]

\begin{tikzpicture}[path fading=fade down]
\node[draw, fill=gray!30,path fading]{Some Text};
\end{tikzpicture}
\end{document}

With result: