\font\gr=grey

{\baselineskip=0pt \lineskip=0pt
\hbox{\gr AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA}
\hbox{\gr AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA}
\hbox{\gr AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA}}

That was a grey region.  Here's some overlapped stuff:

The final test: \rlap{\gr AAAAAAAAAAAAAAAAAAAAAAAAAAA}This is fun text!

\bye