Think of heating a piece of metal. As you add heat, the metal starts glowing a deep red, then becomes orange, to yellow to white (remember that white is the combination of red, green and blue light). So at it's hottest, the metal is emitting all three of these colours.
Now think of yourself. If you added heat under your chair, you might squirm a bit at first, then start to move faster and faster as the heat increases. You would really vibrate at high temperature.
The more heat added, the faster the molecules move, and the blue-er the light emitted.