It's handy to have a bottle of methylated spirits lying around for cleaning or lighting stoves when you're out camping, but it's not fun when someone mistakes it for drinking water. A drop or two of food colouring is all you need to prevent this nasty surprise from happening to anyone.
This tip comes from Brian's Backpacking Blog, which notes that colouring the methylated spirits doesn't impact its efficiency and has the added benefit of making it easier to measure and pour the liquid. You can also label the bottle with a skull and crossbones or use a bottle that can't be mistaken for a water bottle.
Making Denatured Alcohol User Friendly [Brian's Backpacking Blog]