Depends. Unlike solid tumor, there is differet stage, leukemia is usually disseminated disease at diagnosis. For someone without history of leukemia, rash usually is not caused by leukemia (although occasionally leukemia can present this way). For someone with confirmed diagnosis of leukemia, rash could be symptoms of leukemia, or infection, or medication reaction. Check with your oncologist.
Manifestation. This can be skin manifestation of the leukemia. See your dermatologist.