Cellulitis: This is the most likely diagnosis given the presentation of cutaneous rashes and cellulitis on the buttock. Cellulitis is a common bacterial skin infection that can occur in anyone, but patients with myeloma may be more susceptible due to immunosuppression.

Herpes Zoster: Reactivation of varicella-zoster virus can cause shingles, which may present as a rash. Patients with myeloma are at increased risk due to their immunocompromised state.
Fungal Infections: Immunocompromised patients are prone to fungal infections, which can cause skin rashes. Candidiasis and aspergillosis are examples that could present with cutaneous manifestations.
Steroid-induced Skin Changes: If the patient is on corticosteroids as part of their myeloma treatment, they may experience skin changes, including thinning of the skin, which could predispose to or mimic cellulitis.

Necrotizing Fasciitis: Although less common, this is a life-threatening condition that requires immediate attention. It can present similarly to cellulitis but progresses rapidly and can be fatal if not treated promptly.
Malignant Cellulitis due to Neutropenia: In patients with myeloma, especially those undergoing chemotherapy, neutropenia can lead to severe infections, including a form of cellulitis that can rapidly progress.

Sweet Syndrome (Acute Febrile Neutrophilic Dermatosis): This rare condition can present with fever, leukocytosis, and tender, erythematous skin lesions. It can be associated with hematologic malignancies, including myeloma.
Leukemic Cutis: Direct infiltration of the skin by myeloma cells is rare but can cause skin lesions. This would be a rare but significant diagnosis to consider in a patient with myeloma presenting with cutaneous manifestations.