Why rainbow has seven colours only?

"because asian paints cannot afford more than seven colours on such a large scale"

lol..jokes apart.

i can go on with a scientific explanation abt rainbow...its tht the light spectrum splits up into individual components tht is white light is broken into seven component colors

When the sun rays pass through water particles the white color which is made of seven shades splits into its basic seven colors and forms a magical creating called rainbow ..

Actually there are many color but rainbow is actually light reflecting the water precipitation in the air. The only visible colors are seven. When ever both sunlight and rain comes together this is see which tells us that light is main cause.

The light is constituted by the seven colours. It has been proved by lot of experiments. The VIBGYOR is visible in the rainbow since light is scattered by the small particles present in the atmosphere. It only seen when the atmosphere is humid.

It has infinite colors. Red, orange, red-orange, orangish red, reddisher orange, reddish orangish red, vermillion, chartreuse, etc. 
Some say it has less than seven. Namely, not everyone accepts indigo (between blue and violet) as a separate color, so many learn the rainbow has six colors. And a colorblind person who cannot distinguish between, say, red and green will say five or six.