\input luaotfload.sty

\font\testa=file:lmroman10-regular.otf:color=FF0000BB;+trep at 10pt
\font\testb=file:lmroman10-regular.otf:color=FFFF0099;+trep at 10pt
\font\testc=file:lmroman10-regular.otf:color=559922;+trep   at 12pt

\testa \input knuth \par
\testb \input knuth \par
\testc \input knuth \par
\bye