I see the comments about not wanting to make the underpass darker or somehow vandalised, but I don’t think adding some art or identity to that space is synonymous with it becoming somehow dimly lit or subject to graffiti.
All that’s needed is something light, simple or subtle to make the underpass less unremarkable. The already cited example shows how this could be done with a geometric design, though I think Forest Hill should aim higher
The outstanding indented and standout idents @Lionel has drawn under various railway bridges have also survived very well and not attracted more graffiti or vandalism. I don’t think that should be any more of an issue than current maintenance of the area.
So I would welcome some sort of character being added to that underpass, but would ideally like to see a short list of possible designs and public vote to select what might finally be painted.
Of course organising this all on a small budget may prove tricky…