How bright is the Crab Pulsar's 30 Hz modulation in visible light? What color is it?
The optical pulsations of the Crab pulsar have been studied closely since 1969. The observations are actually not that difficult (I did some myself with a photoelectric photometer as a student) and have been achieved with a variety of technologies.

A paper by Fordham et al. (2002) slices and dices the Crab pulsar's pulse shape into fine time and spectral bins in the optical regime. A phase-folded light curve for the full blue-to-red range is shown below. The pulse shape is actually extremely stable and it looks like this in many other papers. The pulse has been "background subtracted" using the signal in the "off" phase of the pulse. The unpulsed component amounts to less than 1% of the integrated brightness in the optical, so essentially "off" does mean off.



The same paper discusses the integrated energy spectrum and the wavelength-dependent pulse shape of the pulsar. The integrated spectrum is almost flat (after correction for interstellar extinction) - as in, if the spectrum $F(\nu) \propto \nu^{\alpha}$, then $\alpha \sim 0$; this result holds for the whole pulse, or if just either of the two peaks are considered (there is a tiny $&lt;1$% variation in the ratio of the two pulses across the optical range). 

Postscript - I notice that the Wikipedia page on the Crab pulsar has a slowed-down animation of the pulse as seen at 800nm (very close to optical). The two pulses and their difference in brightness are quite obvious, as is the "off" phase. It would not look much different at shorter, visible wavelengths.