I know you've thought of this but dismissed it because it's 'ghetto' or 'just stupid'.
You buy a translucent can of dark red/maroon spray paint and dust, DUST not paint the lens. You doa couple quick sweep to where the lens is dusted and you'll get your result. If you buy red translucent paint it will come out pink.
If you can't find translucent paint then just go with a red.
If you mess up you can just take it off right away with some paint thinner.
I've done this mor etimes than I can count wether it be on a faded lens, a clear lens, or a lens someone just wants darker.
A dusting of red rattlecan works perfect. Just take your time and wait for it to fully dry in between dustings and make sure you have no airbourne dust around.
IT WORKS and it's dirt cheap.