I saw a large model which coat the under panel and engine bay with
Rubberize under coating
Something like this...
https://permatex.com/products/specialized-maintenance-repair/body-interior-trim/permatex-heavy-duty-rubberized-undercoating/
It looks very real in my eyes since its colour and texture is on right scale too...
I dont know how they apply it to right scale but it works
Since its the real thing.... so the colour it give is real too