The original color of blood is always red. It could not change but the question is why it looks blue in our veins the answers is that veins look blue because the light penetrates the skin and illuminate the red light and passes the blue light inside the skin because blue light has more wavelengths to go inside ours skin and red light absorbed by our skin.
Most people think that blood looks blue because of oxygen-rich blood coming from the heart through the arteries but that time blood become bright red and without oxygen, blood looks Dark Red.
Blood flow through the heart to arteries and after flows to tiny blood vessels that called capillaries in that moment blood gives oxygen to your body tissues. Lips look the reddest part of your body because it has more capillaries and now blood seems exhausted with oxygen and become dark red while returning back to the heart.