As Nancy indicates, sealing would help if the grout is a type that accepts sealer. You would need to check on that. If you want to change the grout color, there is a grout stain that can be purchased from any professional tile store. You would need to know what grout you used so they can help you assess whether or not it can be stained.

I did my own with a product from Home Depot. Changes color and seals. Did Snow White protects from stains. Poly blend grout renew

I have 3/4 inch grout lines between porcelain tiles. They are my biggest nightmare because as soon as they stain, the whole floor looks dirty. These tiles are throughout my kitchen, dining room, hallway, bathroom and living room. I, too, have used the poly blend grout renew from home depot...buckets of it, actually. Its the only thing I've found that renews the grout. I also seal it with a clear sealer...this process is done every year. Please rethink large grout lines if you are considering them. Your knees and back will thank you.