Great advice dts builders! Another way to minimise visible grout discolouration is to use rectified tiles to minimise the width of the grout join in the first place. Using larger format tiles will again reduce the amount of grout needed, and where your design allows, use a dark colour grout to restrict the amount of dirt and grime that is visible.

Thanks everyone for their comments! Update: husband has 'over-ruled' on this one. Forgot to mention that the vanity top will be Night Sky Ceaserstone (ie black). So he wants the dark grout. We will definitely be using mould inhibitor and ensuring everything is well sealed. It's all his fault if it looks horrible ;) Will post photos once fully complete. (I am now a little scared...)

My suggestion is first talk to the builder to see what instructions he gave the tiler as i'm sure he doesn't want a dissatisfied client. Most tilers will check with the builder before grouting especially when a colour is not specified in a contract. I would then ask if the grout could be replaced, from the looks of it it will only be a couple of hours work. There is nothing worse than having to look at something you are not happy with. I would not use the paint product as it will alway look like it is covering a mistake. White will look better. Make sure the mixer body is waterproofed or you will have a leak in your wall, the membrane should lap onto the body from the wall if it is done well. All the best.

Thanks everyone! Perhaps leaving it is the best option, it is certainly the easiest. I suppose the new owners can change it, and it won't be a deal breaker to sell the house. I think it bothers me because I have a touch of OCD : ). In case anyone is interested, this is what the kitchen looked like before and after: