Possible Mixer Tap Corrosion?

I recently had a new bath,new bathroom washbasin with new mixer tap and also a power shower fitted in my bathroom.
Initially I got rust coloured water from my washbasin mixer tap for both hot and cold settings of mixer tap. At this stage water from hot and cold bath taps was clear.
The plumber then fitted a new plastic cold water tank in the attic as the old one was a galvanised tank and there was some corrosion at the bottom.
Now there is still some rust coloured water from the washbasin tap only. However the amount is less and soon clears away once the tap is left on.
Seems to be corrosion in mixer tap, I can't believe this as it is a brand new part.
Any ideas, suggestions welcome.