The short explanation: dark red, purple, brown, hardened skin is a sign that blood is pooling.
It is one of the early signs of venous insufficiency. That’s what happens when the one-way valves in the leg veins become weak, or the veins become scarred and blocked, so blood flows backward and pools in the legs.
Here’s why the color changes. When blood pools in the veins of the lower leg, fluid and blood cells leak out into the skin and other tissues. Blood cells in the skin, rather than in the vessels, are what you are seeing.
The word early matters. This change comes before any wound exists. It is a sign that comes before a venous ulcer, not a feature of one. The other early signs are itchy, thin skin with faint, light red discoloration, called stasis dermatitis; leg swelling, heaviness and cramping; and itching and tingling.
Hardened is worth noticing alongside brown. The color comes with a change in the skin’s texture, so skin that has become firm or tight as well as discolored is the same thing.
That makes brown skin around an ankle an opportunity rather than only a worry. The measures for venous insufficiency — compression as instructed, feet above the heart as often as possible, daily walking or exercise, keeping the skin of the lower legs well moisturized, and the general measures of not smoking, controlling blood sugar if diabetic, managing blood pressure and cholesterol, and losing weight if overweight — are aimed at this stage.
Discoloration can have other causes, and this page cannot tell you what yours is. It is a reason to have the leg looked at, particularly if it is new or spreading.