Honestly quite a variety. I used Testors aged concrete and weathered black for a lot of it (came in a 3 pen set for military models) and then a variety of red, silver, grey and yellow ones from local art and craft stores for armor damage. A lot of the greys are acrylic which can be hard to use over shiny paint, or certain clear coats. It doesn't want to lay down but wants to puddle up into drops. Anyway silver is almost always oil based which works ok, I use larger tipped pens for large areas and fine tip ones for details and outlines. I've found that if you want to clear coat your damage, you have to redo the silver after as the clear coat tends to dull down the silver sheen.
SchurmanFETT94 keep up the good work!