Grayscale in tikz

Question

I was trying to implement an example from the following answer https://tex.stackexchange.com/a/25708/96546 and encountered some strange behaviour. Here is an example:

\begin{figure}[h]
    \centering
    \begin{tikzpicture}[radius=0.5cm]
        \selectcolormodel{gray}
        \fill [red] (0,0) circle;
        \fill [green] (1,0) circle;
        \fill [blue] (2,0) circle;

        \draw [->,-latex,thick] (-0.5,-1) -- (1,-1) node[midway,fill=red,scale=0.8]{$hello$};
    \end{tikzpicture}
    \caption[.]{.}
\label{Fig::Parratt3}
\end{figure}

While the colors in the \fill commands are all translated to grayscale as required the label of the \draw command remains colored. How can I get this in grayscale too?

Background: I'd like to check if the grayscale version of my picture is still legible, i.e. if the colors can be distinguished.