Blues happen when the soil is acidic and the plant can absorb the proper minerals, like aluminum and iron. In pots, the frequent waterings tend to leech these minerals so you end up having to amend the potting soil to replenish the lost minerals.

The changing of colors could signify that the soil is now less acidic, neutral or alkaline but it also could be caused by blooms that are getting older: the colors normally last for several weeks (or a month) and then the blooms begin a series of color changes that add pink spots or green spots and end in the bloom turning a shade of brown/sand type of color. If minerals are leeching out, the leaves will also respond to that by turning to lighter shades of green or even yellow while the leaf veins remain a dark shade of green.

You can add aluminum sulfate to replenish the lost minerals but, there is not much one can do if the change is due to blooms that are getting old and that start to add pink/green/etc hues to the bloom as it matures. Well, I suppose you could deadhead the blooms or leave them for winter interest although if potted, I would probably cut them in the Fall, when the shrub goes dormant.

Many plant nurseries sell some cheap soil Ph kits that can help you monitor soil acidity levels in the soil or potting soil and that, plus amending the soil to add aluminum (aluminum sulfate), can help maintain the blooms blue but of course, those things will not help once the bloom has matured and decides to start its color changes.

Hello Luis, thanks for that great answer. I bought the hydrangea in its pot nearly 6 weeks ago. Maybe I should use new soil and pot? The pot I bought it in, is not too wide. Do you know how to keep hydrangeas blue without any kind of chemicals? Add another kind of plant?

It is only the soil conditions that result in bigleaf hydrangeas being blue. And if those are not appropriate, only chemicals can alter those conditions :-) However, using chemicals in the garden or on plants is not always to be avoided - water is a chemical compound as are fertilizers, as is the soil it is planted in!! And so are the soil amendments that help to make blue hydrangea flowers blue. As long as you follow the label directions, there is no concern.

ps. I would assume your objection to 'chemicals' relates more to pesticides :-) And that's perfectly understandable. But chemical pesticides are far different from the quite natural means one employs to create the soil conditions required for blue hydrangeas to be blue!!

Thank you for this great answer. Pesticides I don't throw over any plant, because our family works ecologic agriculture. I don't understand people overseeing the risks of using pesticides. You are right H2O is also "chemical". I meant: no pesticides, no fungicides and any other material which hurts the environment. I will try to work on your tip, and use carefully the directions.