New Grout yellowing

I decided to re-grout my whole bathroom due to old grout going mouldy over the bath, and generally discoloured elsewhere.

Got the tools, doing it slowly bit by bit.

Using a very good quality grout from a tile showroom/wharehouse place. Said i wanted white as white can be.

Applied it. Sealed it with Aquamix Grout Sealer.

A week on it is yellowing a bit. Even the bit i haven`t sealed. I am annoyed, loads of effort to re-grout, and it is not staying white.

Any ideas why this is, how to prevent it ? We are in a hard water area, and cannot take the bathroom out of use, it is our only one.

Any advice appreciated,

Ta

DSM

Did you apply bleach? The yellow could be bleeding through from the old stuff underneath.

No, no bleach. I think it is just the fact that the bathroom has no window, and is in daily use, lots of steam from shower. It is acceptable, but just not as bright white as i expected. However, still better than the manky old stuff i`ve removed...