If methane is colourless then why does it cause the blue colour on Uranus?

Answer 1

Nothing is totally colourless. Many people might consider water to be colourless however if you go Scuba diving and go far enough down you will notice that red and yellows get absorbed far sooner than blues. Therefore water is actually a blue colour, it just that you need a LOT of it together in order to see the very slight difference in colour absorption that the substance has. The same is true for methane. In small volumes it is effectively colourless. In vast volumes, such as the atmosphere of a planet, its slightly different absorption spectrum dose make a difference.

Answer 2

The blue color on Uranus is due to the presence of methane gas in its atmosphere. Although methane itself is colorless, it absorbs red light and reflects blue-green light, giving the planet its distinctive blue hue. This phenomenon is known as Rayleigh scattering, similar to the way Earth's atmosphere scatters sunlight to create a blue sky.