`\selectcolormodel{gray}` does not work for `\path[fill=<some pure color>]`

[ghost]

As reported by CarLaTeX in https://tex.stackexchange.com/questions/385158/selectcolormodelgray-doesnt-always-work-with-tikzducks

\selectcolormodel{gray}

does not always work for \path[fill=<some pure color>]

[ghost]

Seems to be a bug in tikz, which above my pay grade, but the bug can be avoided by

• not using \path[fill=...] (instead things like \fill[...])
• a dirty hack and mixing the colour with 0% of another colour, e.g. blue!100
• use the grey option of xcolor, but this affects the whole document, so maybe less flexible...

Related question: https://tex.stackexchange.com/questions/272222/selectcolormodelgray-not-applied-to-draw

Bug report: https://sourceforge.net/p/pgf/bugs/349/

Workaround proposed by Ulrike Fischer: https://tex.stackexchange.com/a/385169/36296