Regarding the white on the chin, I had an interesting thought; check this racer pic out:
Here's one with a lot more white in the chin than the one above, I believe about as much as the one in the original picture:
I wonder if their faces become more melanistic as they age? I noticed that Johnnyb's racer and my racer in the bottom pic both look to be about your average sized racer based on head dimensions (mine was about 3-3.5 feet long), but the one in my top pic has bigger head dimensions and was well over 4 feet long, probably 4.5.
Here's one that was about a 2.5-footer. Lots of white:
I'll pay more attention to this in the field. Not sure when their faces go white though, as they are born without the white coloration under the chin. Here's one that just crawled out from an egg:
Cool stuff!