Aditya, thanks. I like this Lua way, but I have to explore the Lua world first. Ok, reading PIL and coding in Lua.
 Tim> Date: Tue, 23 Apr 2013 10:51:24 -0400
> From: adityam@umich.edu
> To: ntg-context@ntg.nl
> Subject: Re: [NTG-context] Set background color for a single page
> 
> > Aditya, I want to know both please. Now I use the \start \definecolor 
> > ...  ...  \startmakeup[standard] ...  \stopmakeup \stopto assign a 
> > specific color to the tilepage whose content is on a single page 
> > exactly. But I don't know how to set the background for a single page, 
> > say, page 3 if a document has more than three pages. Also, I don't know 
> > how to cycle pages through a finite list of colors. Can you show me 
> > that? Tim
> 
> One way to do this is to set
> 
> \setupbackgrounds[page][background=color, backgroundcolor=\PageColor]
> 
> and define \PageColor such that it expands to an appropriate color. For 
> arbitrary control, you can use a lua function, for example:
> 
> \startluacode
>    thirddata = thirddata or {}
> 
>    function thirddata.getpagecolor(num)
>      -- A lua function that assigns colors based on page numbers
>      if num == 3 then
>         context("blue")
>      elseif num == 5 then
>         context("red")
>      end
>    end
> \stopluacode
> 
> \def\PageColor%
>    {\ctxlua{thirddata.getpagecolor(\rawcountervalue[userpage])}}
> 
> \setupbackgrounds[page][background=color, backgroundcolor=\PageColor]
> 
> \setuppapersize[S3]
> \starttext
> 
> \dorecurse{10}{\input knuth \par}
> 
> \stoptext
> 
> You can also use a lua table to assign colors, and then you do not have to 
> build a ladder of if statements.
> 
> Aditya
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