If we have an EM wave like this one:
$$E=\begin{pmatrix}1\\i\\0\end{pmatrix}e^{-i(\omega t-kz)}$$
The wave has clearly only one frequency $\omega$, but is it monochromatic? My doubt is that it's circularly polarised so the amplitud is changing during time, so the $E$ field is the composition of two periodic oscillations. I don't know if this implies it's not monochromatic, even when there's only one frequency involved, or if it's still monochromatic.