Morning Bob
Having a Pz I track break.
The light effect is loosely based partially on Zenith light which is, if the light is at 12 o'clock, how the shadows fall, lightest at the top darkest at the bottom, not necessarily light in the centre then getting darker to the edge.
Hope that makes sense?
Next ,
If you used the track guards to split a vehicle in 1/2, lower and upper, the modulation on the lower 1/2 goes the opposite way, as it lightest at the hub caps and darkest under the track guards by the body of the tank.
Next
As each flat surface is broken, by a plate at a different angle, the edge can be light on the top edge of th lower plate edge and darker on the other lower edge of the upper edge, usually running vertically, but its more about focusing on the break between plates.
This older DVD does a good job showing you about modulation in a simple way, it might even be on You Tube?
Vallejo AFV painting techniques
As both of my current projects will be a single block colour, Panzer Grey, I'll do one with extreme modulation and one with extreme weathering just for practise and experimentation and the best reason
FUN LOL
Hope its getting a little less like ""as clear as mud""!
Steve H