documentclass[10pt,a5paper,twoside]{report}
usepackage[utf8]{inputenc}

usetikzlibrary{fadings}
tikzset{fading text/.style = {
        left color = {rgb,255:red,0; green,77; blue,87}, right color = {rgb,255:red,22; green,119; blue,74};
}}

usetikzlibrary{external}

tikzexternalize[prefix="/externs"]
tikzifexternalizing{
    % ...
}{
    usepackage{pdfpages} % is needed
}

tikzexternaldisable
tikzset{external/system call={%
        pdflatex tikzexternalcheckshellescape -halt-on-error -interaction=batchmode -jobname "image" "texsource";
        ps2pdf13 -dColorImageResolution=300 "image".pdf "image-13".pdf && cp "image-13".pdf "image".pdf}}


newcommand{fadingtext[1]}{%
    tikzexternalenable
    begin{tikzfadingfrompicture}[name = fading letter]
        node[text = transparent!0, inner xsep = 0pt, outer xsep = 0pt] {#1};
    end{tikzfadingfrompicture}%
    begin{tikzpicture}[baseline = (textnode.base)]
        node[inner sep = 0pt, outer sep = 0pt] (textnode) {phantom{#1}};
        shade[path fading = fading letter, fading text, #1, fit fading = false]
        (textnode.south west) rectangle (textnode.north east);%
    end{tikzpicture}
    tikzexternaldisable
}

begin{document}
    fadingtext{Hello world!}
end{document}