How can I fill tile grout lines so they catch less dirt?

Hi there, Had you thought of colouring them with something like Sharpies?

Seal your grout lines and the dirt will move out naturally.

Here’s a Hometalk replacement link for you to consider. Hope this helps

https://www.hometalk.com/diy/clean/bathroom/how-to-clean-grout-and-install-new-grout-that-won-t-stain-23803774 -grout-and-install-new-grout-that-won-t-stain

It can be done but, it is a little tricky to add new grout over old grout to fill the lines. (If you have not grouted before, it is not the best project to start with.) You would also need to know what color was used in the original job, brand name is also helpful (be aware that the color may not match exactly due to batch numbers, staining, age, and temperature variances). This should not be used to change the color of the grout (due to inconsistencies in the depth of the lines). Make sure your floor is clean particularly the grout lines. Mix your grout to package instructions. Work in small sections as you need to keep the original grout very moist (not dripping wet but very damp) and grout the floor as usual in each section. Allow to set as per instructions and wipe normally. If it pulls out, slow down a bit, wash the haze off of the middle of the tiles and then carefully try to work the lines again. I know some professionals can get the lines level with the floor, I am not one of them, but with some patience you can bring them up to a more reasonable level. After it is cured, seal as desired.