If you put food colouring in a potato seed will the potato grow purple?

that would be awesome

no it might turn blue you didnt say purple food die so it might be pink or green i think you know what i mean

No, if you're only working with the seeds you would have to change the potatoes' genes.
But you might get some effect if you constantly water the plant with heavily dyed water.(or you can by potatos that are already purple)