Red coronary band after shoeing

It is possible that your horse has had a temporary reaction to going barefoot on harder ground, which may have put strain on the hoof capsules. The red line you can see along the coronary band is most likely cell fluid and haemoglobin that has seeped out at the junction with the horn capsule, something that can happen when the hooves are heavily loaded.

Now that the horse has shoes the load will spread more evenly, and this is most likely a passing incident. If the horse is sound and behaving normally, there is no cause for concern. The red line will most likely grow down with the hoof capsule over time.

If you notice further signs of discomfort or lameness, it is worth consulting your farrier or vet, but for now it looks like a harmless reaction to the shoeing. Good luck!