Sorry to hear you're dealing with bumblefoot too! I'm no expert, but I'm currently treating six chickens and have treated others in the past. One of my girls’ bumblefoot looks similar to yours. Sometimes they recover quickly, but other times it takes forever. Also, sometimes the scab can heal over but the infection is still present... I keep removing the scab and rewrapping until you see nothing but normal skin.
Does the foot smell at all? I've noticed that using only vet wrap can make the chickens' feet "too moist," turning greenish and even causing skin rot. Using flexible wrap over a knuckle bandage or cloth wrap helps hold the medication (antibiotic cream and Prid) while keeping the feet dry and clean. Soaking their feet also softens them and can change their color, so make sure to dry their feet before wrapping. It looks like you might be using something "purple." I’ve used "blu-cote" before, and when I wash it, it can look "greenish-yellow" afterwards. Just my two cents ;-) I hope your hen heals soon!