Whilst jno's link is right it doesn't seem to provide a practical way forward. In the UK rather than the US I think you will find that most good garden centres, shops, etc. sell feeds for hydrangeas that will alter the Ph (acidity) and thus alter the colour. However most need to be applied during the growing season - spring to autumn, so you have plenty of time to find one.
Yes sandyRoe you can cut them hard back but not until the danger of hard frosts has passed, usually not until April. This means it is probably too late this year. You might have seen hydrangeas with their dead flowers left on over winter. It is reckoned this helps to protect the new growth from the frost.