Low PAPR performance for Monochromatic frequency signals
The term monochromatic signal is really just a more complicated way of saying a sinusoid, a function that only contains spectral content at one frequency. Peak-to-average power ratio (PAPR) is a bit of a mushy term if you start to look for a theoretical definition, as power by definition is the time rate of delivery of energy over some period of time; the measurement period that you choose will affect the power value that you select.

For our purposes, let's just assume that the "instantaneous" delivered power is proportional to the square of the signal's amplitude (as if it was driving a resistive load). Let's assume the peak power of the signal, then, corresponds with the peak value of the sinusoidal signal $x(t)$:

$$
x(t) = A \cos{\left(2\pi f t \right)}
$$

$$
P_{peak} \propto A^2
$$

To find the average power delivered by the sinusoid, we use the RMS value of a sinusoidal signal:

$$
A_{rms} = \frac{A}{\sqrt{2}}
$$

$$
P_{av} \propto \frac{A^2}{2}
$$

The coefficient of proportionality is the same for both cases, so the ratio is:

$$
\text{PAPR} = \frac{A^2}{\frac{A^2}{2}} = 2
$$

This PAPR actually isn't that bad when compared to many modulated signal types. As an example, OFDM signals often have PAPRs of 10 dB or more, which can complicate the design of amplifiers that are suitable for driving said signals for transmission.