This doesn't look to bad, was the stone sealed prior to the grouting applied? And what with,if so.
We may be able to improve with a light wash with a suitable product and a white pad.
The grout should darken slightly with the application of the impregnator(enhancer), this may be enough.
At the moment the grout in the face also looks lighter, but, if you scrape this out , possibly damaging the stone .what will be replaced in these pits? Dirt!!
Life, indeed will darken the joints and also the grout in the surface, in six months living and maintaining this, you may not see a lot of this and will be easier on the eye.