The previously existing, fully formed and open flowers would retain the original colour, although natural fading or slight alteration can be seen over time - application of substances to alter colour between pink and blue should be made 6 weeks prior to the flower being fully open, and that will be when the buds are forming. That's the latest time to attempt to influence flower colour deliberately. Generally, this response of colour change is only true of Hydrangea macrophylla varieties, though H. involucrata and H. serrata also demonstrate some ability to change the colour of their sepals according to conditions. White varieties cannot have their flower colour altered by the addition of substances to change the ph. More info in the links below
As a point of interest, although we talk about hydrangea flowers, and imagine the typical mopheads seen everywhere, in fact, the mophead is not a flower - the sepals which give the colour of red, pink or blue are a bit like litmus paper, changing colour according to soil ph, or the availability of aluminium, but the actual flowers are tiny, held within the coloured sepals.