Answer:
4/6 chance
One is taken at random, we do not know the colour so we just take away 1 from both colours, this leaves us with 4/6 chance.
As you increase the subintervals the area will be closer and closer to the real value. In other words your approximation gets better.
As you increase the intervals, there will be more rectanagles and the added area of these rectangles are converging towards the actual area under the curve.
