Thats a hard color to try to keep . Maybe someone will chime in with the ph range you need to keep your soil in to maintain that color. The tag is refering to am sun.

Are the stems a ruddy color? Sounds like Preziosa to me. Lowe's is selling a ton of them right now I've noticed.

Another thing I've noticed about hydrangeas is that when stems and leaves are colored like the blooms, it tends to keep the blooms in the same color range in a wider variety of soil types.

Check out this vendor's picture for accuracy: http://www.hydrangea.com/shopping/pgm-more_information.php?id=34