They're fine, that's the way the leaves color up when they shift from growing in low light to high light. They also do this when they experience cool weather. Once it gets hot they will be happier. No need to spray them with anything. Sweet potatoes do best if you just leave them alone - no babying them, no feeding, just a little watering and grow them in poor soil. I don't know where you live but here in Raleigh it's a bit early for sweet potatoes in the ground, the soil hasn't warmed up enough yet. I plant mine in the summer after I've done everything else in the garden and I always end up with way more sweet potatoes than I can eat by first frost.