Ed,
While the search field is in the top header bar, the text colour is set along with the other text in the header bar (in your case: Home Page, Special Offers, Deliver, Contact Us) - which is currently set to white. If you change to another colour, all of the text in the to header bar will change colour.
You could probably change the text colour of the search field only with some CSS code - I'm not very familiar with that, so can't advise - but, a simple solution would be to move the search field to one of the side navigation bars - maybe, just above the your basket?
I hope that helps
