If blood is red, why are veins blue?

I always wondered this, even before I became a sexy vampire. From Live Science:

Blood is red when it has oxygen in it. Without oxygen, it turns blue. So here's what happens:

The heart pumps blood to your lungs to pick up oxygen. The oxygen-rich blood is then pumped out to your body through your arteries. It's red at this point.

From your arteries, the blood flows through tiny blood vessels called capillaries, where it gives up its oxygen to the body's tissues. Your lips have a lot of these capillaries, which is why they're red.

Your blood, now exhausted of its oxygen and turning blue, then returns to your heart through your veins.