Even though grout doesn't take up as much surface area as tile does in a bathroom, if you have problems with your grout it can become quite an eyesore, making the bathroom tile seem practically invisible in comparison. For example, if your grout has mould and mildew growing all over it, resulting in massive discolouration, your bathroom could look pretty rough. Luckily, touching up your grout is rather simple and doesn't require a contractor or handyman.
Pour 1/4 cup distilled white vinegar into a clean, empty spray bottle and fill it with cold water. Replace the cap and shake it well.
Spray it generously on your grout, saturating it as well as you can. Scrub in between your grout with an old toothbrush. This will help you remove as much discolouration as possible before the touch up, not to mention disinfect any mould and mildew that's there.
Rinse the grout and tile completely with cold water.
Shake your grout touch up pen well. Start at the top of the first column of tiles. Run the pen along the vertical sides of the tile, creating one straight line. Repeat this until you've coloured over all your vertical grout lines.
Shake your grout touch up pen again. Place the tip of your pen along the far left side of the tile, at the first horizontal line of grout. Drag your pen evenly across, creating a new white line and covering up all the discolouration. Repeat this with all the horizontal grout lines.
Allow the grout colourant to cure for 24 hours before using your bathroom.