Highlighted or highlit?

I wanted to ask the Zoe covid app people why, when they are talking about Argyll & Bute, the part of the map of Scotland with the most highlighting on it, is not Argyll & Bute.

So I've now asked them, using your wording ?

Thank you, oldnproud! You have hit the spot. What you said describes what I mean perfectly.

Neither - I would talk about the part with " most highlighting" on it, but that's just me.