There’s two main ways to get white light from LEDs:
The first method is to combine light from red, green, and blue colored LEDs. If you get the right mix, the effect is white light. This is the same way your television works—a white object on the screen is really depicted using dots of red, green and blue lit up in proportions that form an impression of white. White formed this way can be “tuned” to look warm or cool by adjusting the amounts of colors in the mix.
The second method uses a blue LED with a phosphor coating. The coating emits a yellow light when the blue light from the LED shines on it. The mix of the yellow light with the blue light forms a white light. Inefficiency in the phosphor conversion is one reason that a white LED is less efficient overall than a colored LED. Some of the light energy is lost in the conversion to yellow.