[samthekiwi (OP)]

Hello,

I am currently working on a liquid cleaning product and I want it to be blue. The liquid has a PH of 3-4 meaing it is mildy acidic. I have tried food colouring and it will change the colour to blue however overnight the acid in the liquid is then changing it back to white.

Can anyone recommend a colouring ink or dye that will work with an acidic liquid?

Cheers,
Sam

[Bored chemist]

https://en.wikipedia.org/wiki/Methylene_blue
is fairly acid stable.