Hi Hala, I have this problem no matter what you do the grouting always yellows with time especially noticeable against white tiles, but you can get a touch up grout in DIY stores it comes in tube form and ready mixed and normally with a sponge applicator, it's very easy to apply.
Hope this helps. The crafter.
I use Milton and find this seems to work. I used it a lot when we had our mobile caravan as over the Winter the white areas always seemed to lose their sparkle. Guess Parazone would be cheaper over a large area.
Curiosity made me Google the Parazone idea and I found the following: