The question might sound naive, but the answer is actually not entirely straight-forward. It is easy to think that red wines are made from blue grapes and white wine are made from green – but there is much more to the wine making process than just that.
The colour of the grape comes from its skin. There are green grapes and blue grapes, but the juice is always close to white. When making white wine, the grape is crushed, the juice extracted and then fermented into wine. When making red wine, the grape is crushed and then the juice is fermented together with the skin. The intensity of the wine will depend on how long the skin is stored together with the juice and how thick the skin of the grape is.
This means that it is fully possible to make white wine from blue grapes, but impossible to do the opposite. Champagne is a good example: In the Champagne region grows two types of blue grape (Pinot Noir and Pinot Meunier) and one type of green grape (Chardonnay). Despite this, almost all champagne is white. You simply crush the grapes, extract the juice and discard the skins. This way, the must has no colour from the skins, and the wine is white.