I have just tiled the splash back in our cloakroom with small rectangular tiles in a brick bond pattern. The tiles have a handmade look (i.e. they are slightly "crooked/wavy") and are glazed on a dark terracotta colour base. The tile edges are only partially glazed, in some places not at all. The tile surface is a glossy light grey. Overall the job looks great, now that it is grouted. The only thing that really bugs me is that all over, you can see just a hairline of the underlying terracotta base in places, where the tile edge glaze is incomplete or translucent. The grout does not hide it fully - to achieve this, I would have to apply the grout so deep, it would be level with the tile surface and that would just look weird. 

How can I overcome this problem? The red lines spoil the overall grey, understated colour scheme. Can I apply more grout over the existing hardened grout? It was done a week ago and is still clean (no grease etc). Thanks. 

There are white pens, intended for re-whitening grout.  a bit like tippex correction pens.  I think that would stick to terracota.