How do I fill missing grout on floor tiles easily?
You can buy a 1lb. container of grout for $6.00 at the big box stores. You should be able to get a color that is close.
I believe you can buy squeeze tubes of grout from tile department in DIY's.
Grout isn't expensive. The price is higher, the more convenient and prepared it is. If you are anxious about the process, watch some YouTube videos to get comfortable.
Here’s a Hometalk link for you to consider. Hope this helps