The most famous experiment on ph and colour is the red cabbage  extract experiment. Very wide range of colour  depending on ph. 

Basically cell sap ph is independent of soil. Ph of soil can effect plant uptake of chemicals and minerals, so can effect  colour of flower, but not much effect, if any on cell sap ph, which is controlled by plant genetics.

Each population of plants (ie  iris species  etc) will have it's own mean and standard deviation.  AS iris have derived from a number of species there could be  a wider range of cell ph then in other species. But this has never been checked, as far as I know.  But there is a lot of experimental evidence that cell sap ph  can make a huge difference.

For example Bachelor Button is very blue, but it's anthocyanin is  cyanidin. the red  pigment in roses.  It is the cell ph  which results in it's blue colour.

Chuck Chapman