I use a bottle of BioBor mix that I have added dye to, this then dyes the fuel. Since I always add the BioBor and oil at the same time I am covered.
Search for Kero dye, there are lots of places that sell dyes of any color to color the kero in decorative oil lamps. You need VERY LITTLE to dye the fuel.

For the BioBor mix I use 1 oz BioBor to 15 oz of fuel and a few drops of dye. this mixture is then the recommended dosage for 1 oz to treat 5 gallons of fuel. I use one of the bottles that has the built in one oz measuring cup for measuring out the dose for the 5 gal fuel jug.

If you are using jugs of oil you could add the dye right to the oil jug.