Why are the night-blooming flowers white in color ?

They are less colourful because they cannot be seen so well, so colours would not attract pollinators as teh colours of daybloomers do. I'd say they are often white because white stands out best in the dark, so is best for attracting creatures. Night bloomers such as jasmine as also quote often scented for the same reason, they rely on this rather than colour to be pollinated.

that is because they need less chlorophykk because there is no sun light. you shoul do this for night blooming plants. For best plant performance, it is desirable to match the correct plant with the available light conditions. Right plant, right place! Plants which do not receive sufficient light may become pale in color, have fewer leaves and a "leggy" stretched-out appearance. Also expect plants to grow slower and have fewer blooms when light is less than desirable. It is possible to provide supplemental lighting for indoor plants with lamps. Plants can also receive too much light. If a shade loving plant is exposed to direct sun, it may wilt and/or cause leaves to be sunburned or otherwise damaged.