Wouldn't you know it - I'm seasoning my cast iron at 500°F now (as opposed to the 375° I was trying earlier) and it's turning the proper color.
Come on, polymerization!
@voltronic You should always start seasoning at a lower temperature, say 200F or so for a few hours to help fill in the pores on the metal. After that you can continue with the higher temps or just start cooking with it. We continuously reseason with a tsp of oil brought to smoke temp and allowed to cool gradually after each cooking/cleaning session or two. It has kept our cast iron in good shape for decades.