The simplest way is probably to add a Wall Shadow, Point Light Finalize sheet effect to one of the last sheets in the list, check the Shadow Color is ambient light box and pick a nice bluish colour. That way you can turn the effect on and off.

That is what the colorize effect does - sets the colour.

The HSL effect works with the colours as they stand, so that if you have a blue rooftop you can change the hue by increasing or decreasing it around the colour circle, increase or decrease the saturation of it (that one is the wrong way around for my way of thinking, since negative numbers increase the brilliance of the colour). Lightness is the easiest of the controls, being reasonably self explanatory.

So whatever colour your roof is, these settings would make it 5% further along the spectrum (for example if you start with blue it will go more violet). With the Saturation set to -10 that will make the colour of things on that sheet 10% more garishly bright, and the Lightness value will make them darker by 30%.

This is an adjustment, rather than a setter. It's like messing with the settings on your monitor.

You should be able to use any of the colour altering sheet effects on the symbols. This is unique to city maps.

Changing the colour can add interest, or mark special buildings

No. There are only 8 steps in 100, so every 12.5 units is a step.

You can get more subtle effects by setting the Strength to zero and duplicating the effect.

Ralf covered this in a relatively recent live mapping session here.

Welcome to the forum bakrin :)