Can cellulitis cause skin discoloration/bruising? Or should I seek emergency assistance?

Yes. Cellulitis will cause swelling and redness and and on the legs or deep involvement can have a dark color. This needs treatments with antibiotics either orally or in more severe cases with intravenous antibiotics. See your doctor as urgent appointment today if possible. Otherwise go to er.
Cellulitis. Cellulitis cause the affected skin to turn red, get hot, slightly swollen, and tender. Untreated, it can spread & become very serious, so get it treated.