Generally when we say 'blue spray' we mean tetracycline (antibiotic) spray. Our vet stocks two variants, one called Terramycin and one called Cyclo Spray. The former is quite a bit more expensive and the only difference is the propellant and the size of the hole the spray comes out of!
Having conducted experiments, we still pay extra for the Terramycin. Our opinion is that the narrower 'beam' results in more effective delivery and less waste. It's quite subjective though, so we may just be suckers!
When I say 'purple spray', I mean Septiclense. It's an antiseptic, not an antibiotic, and I prefer to use it if there is no active infection which needs antibiotics. It is cheaper but also we prefer not to overuse antibiotics.
There is also a variant of purple spray called Footclense. It has a couple of added ingredients specifically for footrot and scald, and I think probably is slightly more effective on sheep feet than straight Septiclense. However, we tend to use either Septiclense or blue spray.
Hope that helps rather than thickens the fog!