LaTeX - Making a lighter version of text, like anti-bold?

If you're interested in making just a small section lighter, you could use

\usepackage{color}
\definecolor{light}{rgb}{0.5, 0.5, 0.5}
\def\light#1{{\color{light}#1}}

then wrap some text you want to make lighter in

\light{some text to make lighter}

\font\tenrm = cmr17 at 10pt
\tenrm

Note that \fontseries doesn't define what's available for any given font -- it accepts anything in the font definition files for the font family. the set is defined in the document fontname (ctan info/fontname), and definitely does contain light.

so the complaint is that fonts that the user has encountered either don't have light weights, or whoever wrote the .fd file didn't include it. that knuth's (metafont) fonts contain an extra-bold weight by default, but not anything light, is something to go away and think about.