How to add realistic grunge to crevices I'm modeling BB8, and having difficulty getting really good dirt and grunge on him in the right places and the right texture.  I did watch Andrew Price's video on how to add grunge using dirty vertex colours but as you will see the effect I'm going for is not quite the same as what he did.  I'm trying to get precise, accurate dirt that really looks like something has been rolling on a dirty surface for a while.  How can I achieve this?

Here's the reference photo I have been working off of:



Here is the render:



Here is the vertex paint settings:



Here is the node group I made for the grunge (it is plugged into the principled node):



And not sure if this helps but this is the grunge image I was using (colourized with brown in GIMP):



Here's a link to the .blend (was too big for giant cow films): https://drive.google.com/open?id=1edSlMISPUr0KYetTipJ9FHTacljA2am3
Okay I think I may have found the issue?  Where you have an Add node I think you really want a Multiply node, as follows:



And the render looks more like this:



Which seems more like your reference photo I would say.