\input luaotfload.sty
\font\testa={name:Latin Modern Roman:color=0000ff99f} at 10pt
\font\testb={name:Latin Modern Roman:color=0000ff9}   at 10pt
\font\testc={name:Latin Modern Roman:color=0000f}     at 10pt
\testa test\par
\testb test\par
\testc test\par
\bye