Being treated for cellulitis with antibiotics, today my swollen legs have started to look bruised with a blue/purple tinge. is it getting worse?

It depends on sympto: If the antibiotics are working for you the symptoms of cellulitis(Pain and swelling) should diminish within 24 to 48 hours of starting antibiotics(The color change may not be a good guide but the change in the severity of pain is the key symptom to guide you). If you start getting fever, then it is a bad sign, indicating worsening of infection. Then go back to your doctor for a check up.

Unknown : You need to be seen by doctor ASAP. May be better may be worse. Need to see these.