Why red grapes are only used for making red wine?

Answer 1

The color of the grape determines the color of the wine - thusly red grapes produce dark red Grape Juice which, when fermented, retains the red color.

Green grapes produce a clear juice which remains relatively clear through the fermentation process.

Blush wines are commonly a mixture of both types of grapes.

There are wines which are made using other fruits - and those wines retain the color of their fruit juices as well. Blueberry wine, for example, stays dark blue.