_1psi_1 left[begin{matrix} a \ b end{matrix} ; q,z right]
= sum_{n=-infty}^infty frac {(a;q)_n} {(b;q)_n} z^n
= frac {(b/a,q,q/az,az;q)_infty }
{(b,b/az,q/a,z;q)_infty}

documentclass{article}

usepackage[dvipsnames,usenames]{color}
usepackage{amsmath}

defzz#1#2#3{%
begingroup
lccode`~`#1%
lowercase{endgroupedef~}{#2{#3}}%
mathcode`#1"8000
}

begin{document}
zza{}{noexpandtextcolor{red}{mathcharthemathcode`a}}
zzb{}{noexpandtextcolor{green}{mathcharthemathcode`b}}
zzq{}{noexpandtextcolor{Mahogany}{mathcharthemathcode`q}}
zzz{}{noexpandtextcolor{yellow}{mathcharthemathcode`z}}
zzn{}{noexpandtextcolor{Aquamarine}{mathcharthemathcode`n}}
zz=mathrel{noexpandtextcolor{green}{mathcharthemathcode`=}}
zz-mathbin{noexpandtextcolor{YellowGreen}{mathcharthemathcode`-}}
zz1{}{noexpandtextcolor{cyan}{mathcharthemathcode`1}}
zz,mathpunct{noexpandtextcolor{Sepia}{mathcharthemathcode`,}}
zz/{}{noexpandtextcolor{Tan}{mathcharthemathcode`/}}
zz;mathpunct{noexpandtextcolor{blue}{mathcharthemathcode`;}}
letoldinftyinfty
definfty{{textcolor{PineGreen}{oldinfty}}}


pagecolor{black}
color{white}

zzz
begin{equation}
_1psi_1 left[begin{matrix} a \ b end{matrix} ; q,z right]
= sum_{n=-infty}^infty frac {(a;q)_n} {(b;q)_n} z^n
= frac {(b/a,q,q/az,az;q)_infty }
{(b,b/az,q/a,z;q)_infty}
end{equation}

end{document}