You know I thought it was obvious - but in case it's not: The original photo is of the GW painted rhino; I used 'photoshop' to demonstrate my weathering technique. I added NO real paint to this model. :) :)
The basic technique is this:
step 2) create blotches, especially on corners and edges in a dark color (Scorched brown works great)
step 3) find the lightest/brightest highlight tone of your 'base' color. Use this to accent the areas of the chip that will reflect the most light - typically the bottom... so brown on top, highlight underneath.
step 4) add boltgun metal to lower half of damage and hard edges. From here you can further add mythril silver highlights to the most reflective spots but it's not necessary.