OK, after digging around some more I was able to answer my own question... it turns out, there is an icon at the top of each source window for Document Settings... click on that, and you have options for file format and syntax coloring. It's the fourth icon from the left, the one that looks like a piece of paper with a checkbox list. Here is the thread that pointed out this feature:
Re: Color Syntaxe for ASM code
I have no idea why some of our source files had this turned on, and others did not. I have turned on the feature for all the files that lacked it, and life is good.