It's true that the carrion crawlers' background is yellow rather than the same shade of green.
I'd say that the slaad, however, does sort of fit the "background is a lighter shade of the text" - pink is a lighter shade of red, after all.
With two different "fiends from hell" both of which have the red-and-yellow combination, maybe that's the default for Baator now - red, yellow, or both? Qaar was red on black, Lee was yellow on black, and we have at least two red-on-yellows.