Hope you find it useful :)
Don’t forget to post your results in the comments below.
1. Select the model you want to add dirt to.
2. Go into Vertex Paint mode (V) and in the menu, select Paint>Dirty Vertex Colors.
3. In the toolbar (T) change the blur strength to 0.5 (the lower the value, the sharper the effect).
4. Change the highlight angle to 90 (this basically acts as a threshold – the lower the less dirt will appear all over your model).
5. In the object panel, find the new Vertex Color entry and name it something useful like Dirt Map.
6. To use the Vertex color data in your material, go to the node editor and add an Attribute node (Add>Input>Attribute). In the name field, type the name of your Vertex Color group (Dirt Map).
7. Add a MixRGB node and connect the Fac output of the Attribute node, to the Fac input. Connect your base material to the bottom input.
8. Change the color of the middle input to whatever color you want the dirt/grime to be (green or brown works well).
Render now and you’ll see a faint green tinge buried in the crevices of your mesh. Finished!