I have high blood presure an when my legs swell they turn red why ?

Typical: When you retain fluid in the legs, the skin flushes and looks pink. A lot of times this is mistaken for cellulitis, a skin infection. The bigger issue though is why are your legs swelling? It's not normal in hypertension unless something else is going on. So see a doctor to find the cause.

Inflammation: You may have leg swelling from diastolic heart failure associated with uncontrolled blood pressure or fluid retention due to incompetence of the valves in the veins of the legs. In either case stretching of the skin and soft tissue of the leg can cause inflammation and/ or infection resulting in redness.