[Dr.Sid]

I'm not really expert but this was discussed earlier here.
The thicker lines are blade frequency. It's always dominant. You see more that one because what you see are harmonics - multiplies of base frequency. Practically any sound has such harmonics. So if you look for you real blade frequency, it's the lowest frequency (23 Hz on green, 45 Hz on black & white).

Then those thinner lines between those thick lines are harmonics of shaft frequency. Either shaft makes these noises or there is difference in individual blades, so there is something which only can be heard once per turn.
Again, the lowest is the base shaft frequency, the rest are just harmonics. But what is important is how many times the blade frequency is larger then shaft frequency, or (in other words) how many shaft harmonics fits between blade harmonics, because that's how many blades the screw has.
You can see it's 3 on b&w image and 5 on the green one.