I don't think the "highlight" is intended to patronise the designer, by suggesting some symbols might be beyond his understanding. It would be very weird if some symbols were not filled because they are considered "basic", e.g. 555. 741, every one knows those.
I thought you might have some sensible reason, like "it's hard to read with a yellow background" or something. Unless there is some objective reason, I would rather follow a consistent style.