Hi, I agree with Hellybee - seing the lamb to see the vet is your best chance of making sure you don't lose the lamb and making sure the lamb's welfare is not unacceptably compromised.
But I know from prior experience that not everyone will follow this advice, so I'll just add that if you can see a track leading up towards the spine like that, a decent course of injectable antibiotics (which some vets will supply to you after a telephone conversation if they are happy in their clinical judgement that this is the best course of action) would be a good idea to prevent a spinal abscess developing (it will be fatal if it does, so it's best to try to prevent it if you can). Good camera phone pictures can also be of use to vets in clinical decision making as to whether antibiotics alone are going to cut it.
Consider whether the lamb is vaccinated for tetanus as well (your clostridial vaccines, if used), if you normally vaccinate your lambs a wee bit later then you might want to just bring it forward to avoid losing a lamb (if you don't normally vaccinate, don't worry, perhaps just mention this also to your vet one the phone to see whether it is worth it for you in your particular situation)
Hope this helps, if it's really bad then as I say, welfare wise and outcome wise then direct treatment of the lamb your vet is the best bet, but if you feel that this is not an option for you at present then do give your vet a wee telephone call to see if you can get some antibiotic cover. Purple spray is too superficial if there is a track heading upwards.