Here is the "color sensor". In order to distinguish the "color sensor manufactured by HiTechnic" it is call the "color sensor manufactured by Lego."
The color sensor is appeared so relatively new, not included in the basic set of "Lego Mindstorms NXT for Education." It is also sold separately.
It does have the 3 round windows, two are full color LED and light sensor. When the objects of light (red, green, and blue) are determined by the amount of the color came back from LED with the three colors.
The color sensor can be switched by operation mode, using a "light sensor" and "colored lamp".

So, let’s see what happens.
Connect the color sensor to port 1 on the NXT intelligent block.

Put the power of intelligent block NXT and select "VIEW" → "Color". The name of the color with sensor detection will be displayed on LCD.

Color sensor can detect six kinds of colors in total. Those colors are numbered 1-6 in color. For example, when detecting the green is displayed as "3.Green".

In some cases, the color sensor could not detect the color.
When you measure the middle of the block of "green" and "red", for example it is displayed "Yellow (yellow)".
So it did the erroneously recognized as "yellow", by reflecting that the amount of "green" and "red" have increased. And "green + red = yellow" in the rule referred to as "the Act allowed color."

When you measure the middle of the block of "yellow" and "blue", similarly it is displayed "White (white)". According to "the Act allowed color ", it becomes the "yellow +blue "is "white".
If there are three primary colors of red, green and blue, the any color could be produced in theory. For example, it has been made in the three primary colors of red, green, and blue color of the TV screen.