Hi Yui. Blood in humans is always red however it can become redder when it touches the air. The reason blood is red is because it contains Iron in the blood cells in a structure called haemoglobin. This structure is designed to catch and hold onto oxygen allowing it to be transported around the body! When this structure catches oxygen it causes it to change shape and this change in shape makes the blood itself look redder. So when blood touches air it sucks up all the oxygen it can get which makes it look redder.
Not all blood is red in the animal kingdom, for example Horseshoe crabs (which live in the ocean) have blood blood becaus insteade of using Iron they use copper in thier blood to catch oxygen.