mjhjmtu

Right now, I am generating a random color in pdftex using

definecolorrandomcolorRGB

 pdfuniformdeviate 255,
 pdfuniformdeviate 255,
 pdfuniformdeviate 255

Which works pretty well,
until I try and compile with luatex.

What is the nicest way to generate random colors (either directly or via generating 3 random numbers), that is compatible across both pdftex and luatex?

I suspect something involving pgf is the answer.
Or maybe just a condition with one branch for pdftex (that looks like the current), and one for luatex (that maybe invokes a tiny snippet of luacode).

The luatex85 package does

letpdfuniformdeviateuniformdeviate

You can thus do usepackageluatex85 or

documentclassarticle
usepackagexcolor

unlessifdefinedpdfuniformdeviate
 letpdfuniformdeviateuniformdeviate
fi

definecolorrandomcolorRGB

 pdfuniformdeviate 255,
 pdfuniformdeviate 255,
 pdfuniformdeviate 255

extractcolorspecrandomcolortest
typeouttest

stop

This will type out something like

rgb0.84706,0.4,0.68234

with both pdflatex or lualatex. I guess that, when random numbers will be available also in xelatex, a common interface will be added to the LaTeX kernel.

Based on Randomly assign background colour for each frame

Another approach based on pgf:

documentclassarticle

usepackagepgf
usepackagepgffor
usepackagexcolor

usepackageifluatex
usepackageifxetex

ifluatex
 letpdfrandomseedrandomseed
fi

ifxetex
 pgfmathsetseedtime
else
 pgfmathsetseednumberpdfrandomseed
fi

newcommandrandomcolor%
 pgfmathsetmacroRrnd%
 pgfmathsetmacroGrnd%
 pgfmathsetmacroBrnd%
 definecolorrandomcolrgbR,G,B%


begindocument

noindent%
foreach n in 1,...,1350%
 randomcolorcolorrandomcolrule0.41cm0.41cm-%


enddocument

There is also the lcg package (short for Linear Congruential Generator), that works with all compilers, including lualatex. The package provides a command rand to generate a random number and store it in the rand counter. For use in definecolor you should convert the counter with thevalue.

You can set the bounds of the random generator with the package options first and last, and optionally change them within the document using the reinitrand command. The default seed is based on the system clock, using the minutes as unit (so the output will change only once per minute). You can also provide a custom seed.

MWE:

documentclassarticle
usepackage[first=1,last=255]lcg
usepackagexcolor
randedefcolorathevaluerand
randedefcolorbthevaluerand
randedefcolorcthevaluerand

definecolorrandomcolorRGB

 colora,
 colorb,
 colorc

begindocument
colorboxrandomcolorRandom color: colora,colorb,colorc
enddocument

Result: