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You can fairly easily change a grout color if you go from light to darker. You basically just paint it on. It's more like a stain than paint. Here's one option.https://www.amazon.com/Aqua-Mix-Grout-Colorant-Charcoal/dp/B000GFJTSI
Since the brown is also a dark color it might work but you'll need to test a small inconspicuous spot to see if she's OK with the result. No way a lighter color will work over a dark color. This is stain it's not paint.