For most of us, seeing colour is pretty easy. The Sky is blue, the sun is yellow, the ocean is deep blue, the Steemit page is white.

Except all of that is wrong: The sky is clear, the sun is white, the ocean is clear and the steemit page is Red, Green and Blue. (I can explain each of those in the comments if you want)

Clearly human eyes and brains are not reliable when seeing colour. So how on Earth do planetary scientists tell us what colour exoplanets are, thousands of lightyears away?

What we need are tools.

Pointing a telescope at a planet is not enough when you're looking for a tiny dot quadrillions of kilometers away (though this IS a technique with the right glare-blocking tools). There's a whole range of superior techniques and technology needed to get a proper look.

The Transit Method (2734 planet discovered)

This is probably the most known method and seems to be the easiest method, having found almost 3,000 planets to date this way. Rather than trying to look for a tiny body that doesn't emit any light, it's much better to stare as the star it orbits. When you observe a dip in luminosity, that's not because the star is dying, but because something is in the way, blocking some of that light. If this happens at regular intervals, you have yourself an orbiting planet.

Then you can get a good measure of its size by how much the light dims, and composition by observing the light that passes through any of the planet's atmosphere.

To do this, scientists use:

Spectroscopy

Without wasting too much time getting sidetracked, each gaseous element has a kind of light signature. Helium, for example, emits a certain variety of light when heated up, and different elements have their own unique fingerprint.

Below you can see the brighter vertical lines are the absorbed wavelengths that we don't see for each element, and the rest is emitted to us. So we can pretty reliably get an idea of which elements and molecules are in the atmosphere this way

Radial Velocity (643 planets discovered)

As mentioned previously, planets actually orbit the barycenter, the shared point that stars and planets orbit. The bigger the planet, the further from the sun's centre the barycenter is.

For big planets, it's quite easy to use this to our advantage. By using dollpler shifting of light, scientists can observe the difference in movement the star has; when the star is moving away, light waves are 'stretched' and are viewed as redder light. when it goes towards us, that light is squeezed together and become more bluish.

Think of the planet in the below image as an ambulance rushing past you. When the ambulance is coming towards you, the pitch is higher because soundwaves are scrunched together. When it passes by, the soundwaves are stretched out so the pitch becomes lower. The same applies to light.

Miniscule movements

Another similar method is to simply observe the wobble in relation to other nearby stars. By measuring the tiny differences in distance between two or three other stars as the main orbits its barycenter, you can accomplish the same reults

Gravitational microlensing (47 planets discovered)