You could either create a "dummy" narrator NPC that isn't seen that allows you to set a color as you would any other NPC, or create a separate Subtitles menu that only shows for Narration text.
The For speakers of type field in Subtitle menus allows you to choose which speakers it appears for.
I want the Narrator used when Looking at my Inventory, and I don't want to create a dummy agent in every scene.
I already have a separate subtitles menu for the Narrator only, but since he's not an actual NPC, I have no place to set subtitle color for him, so it overwrites my menu defined color (yellow) with the default white.
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The For speakers of type field in Subtitle menus allows you to choose which speakers it appears for.