On Saturday I went down to the local Canadian Tire (hardware store) since they've actually got a decent selection of spray paint. As recommended on numerous sites, I found the styrofoam friendly Krylon H2O Latex paint. I quickly snatched up cans of red and blue, but hit a snag when they didn't have silver. They did have grey, and after some though I figured that in the Transformers cartoon, Optimus Prime had a grey grill. Sure silver would look better, but it'd also eat up styrofoam unless I could find some sort of styrofoam safe kind. Besides. I can always paint over it later should I find some decent silver.
With the three cans of paint ready to go (and surprisingly no sales people thinking I was a conscientious vandal who uses water based paint so his work can be cleaned up easily), I sacrificed my Roomba's box to make a spray booth. Come on. You think a guy doing all this work for an Optimus Prime costume would have any other vacuum cleaner besides a Roomba? Pft! (Seriously, I highly recommend them.)
With even sprays, I covered the forearm. The pen marks still showed through but I think a second coat will take care of that. After it dried, I was pretty happy with the colour. Not too dark at all. I have to admit, up until this point, it was looking like a bunch of pink styrofoam. Adding colour really seems to give it new life.