I would hypothesize that it has to do with soil acidity... Hydrangeas bloom white, purple, pink or blue depending on soil acidity... maybe the hyacinths are like that too?
This is just a THEORY... I don't know for sure, but it sure is interesting.
(come to think of it, flamingos also change color depending on what they are eating!)

Sometimes temperature has a lot to do with color in roses. I have one (Aloha) that blooms deep salmon in cooler temps and clear pink in warm weather. Maybe hyacinths work the same way?

I also have deep raspberry hyacinths that are blooming a light pink. It is the first year they are doing this & it is also the first spring that has been so cold. For a couple of weeks now we have had colder than normal temps. I choose to think next spring they will be normal in color again. Although my blue & white hyacinths are normal in color.