There are grout pens just for a project like yours. Otherwise you could remove the old grout and redo it.
It can be dyed or stained.
To change it is a lot of work, it requires digging it out with a grout cutting tool.
Hi Tanya! Here's a great tutorial on how to change grout. https://www.doityourself.com/stry/how-to-repair-grout-around-the-bathtub
Tanya...I redid my kitchen grout when I realized I had gone too light. This was years ago and still looks great. The pen that Cynthia mentioned would be great I'm sure. I did mine with a toothbrush and damp rag for overrun. Her info is probably easier! Good luck!
They have grout pens that would work on changing the color of the grout
Hello. Heres some links that might be helpful.
Painted Tile Grout Refresh with grout pen
Grout cleaning and grout removal and replace it
Here’s a Hometalk link for you to consider. Hope this helps