If あなた put 食 colouring in a potato seed will the potato grow purple?

that would be awesome

no it might turn blue あなた didnt say purple 食 die so it might be ピンク または green i think あなた know what i mean

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