I like to use tile & grout cleaner (a bleach gel) to clean the grout in my shower. However, the tile & grout cleaner is white, the grout is white, and the tiles are white. How can I easily tell where I've applied the cleaner, and if I coated the grout thoroughly without missing any spots?
Add an organic dye to the bleach gel just before application.
It would guide you in placement and be bleached away in time (seconds, minutes, hours) depending on bleach and dye in question, any residual should be rinsed away.
I would start with food colouring and work to inks and then more interesting stuff like organic pigments.
For a gel type bleaching product that you need to make visible I would use a contrasting coloured powder that you blow/shake or dust on to the gel during application. You could use household products like corn starch for black tiles and cocoa powder for light tile.
You could use a salt shaker, a cloth bag or hem marker.
After application you rinse off gel and dust.