As title, could anyone kindly advises me how to paint realistic battle damage on AFV's armor?

Do not know if this is too extreme, but issue 9 of The Weathering Magazine dealt with it:
http://www.migjimenez.com/en/english-version/305-issue-9-ko-and-wrecks-english.html

Thx mate! I will check it out!

I can only tell you what I did and it worked out very well. Burned vehicles don't look like you imagine. I went online and looked at color photo's of burnt out tanks. I chose a tank that had similar damage to what I wanted. I then looked at the photos artistically to determine what colors I would need and any effects like burnt rubber tire ash etc... I looked closely at the color layers to see what colors would be a base color, a mid level color and a surface color. You need to layer the paint like a landscape artist. There were many colors in the burned area. I did my damage first, Then I did the ash where the rubber burnt. I then started painting. The final product was beyond what I expected. For damaged metal I have used aluminum foil and pressed it over the part I wanted to damage. I then carefully trimmed it and "carefully" damaged the foil copy and used the foil copy after removing the original part. Don't use a very thin foil because it is too delicate.