Problems with conversion of `Unsigned` to `RGB`

I've accidentally been using `Base.convert` method from this package that converts `Unsigned` to `RGB`. This is type-unstable (which is easy to fix) but it also looks like type piracy, so this method likely shouldn't even be in this package. Here is that method: https://github.com/JuliaPlots/PlotUtils.jl/blob/44e45ec395afb6eaf84592f72247cdba30409546/src/color_utils.jl#L96-L99 . What do you think should be done about it?

Start with src/color_utils.jl at the linked lines containing the Base.convert method and inspect how Unsigned-to-RGB conversion is used in PlotUtils.jl. Clarify with maintainers whether the method should be removed or relocated, then verify the chosen change against the package's existing conversion behavior.

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