[patriciaruth]

Methylene blue. (Ask a clinical lab to supply you with a Gram stain kit).

Or ask a pharmacist for the purple stuff they used to paint throats with back in the old days.

These are dyes that won't kill your evil twin neighbor, just bust him.

[The_Victor]

Or ask a pharmacist for the purple stuff they used to paint throats with back in the old days.

That would be gentian violet. Should be available over the counter.