Hello and thanks for the question! We see colors due to waves of light so the reason why certain planets are different colored can be explained by what they are made out of and how their atmosphere reflect or absorb the light from the sun.

Mercury has a dark grey and rocky surface, Venus is covered with carbon dioxide and sulfuric acid which makes it a light yellow color. Earth as we know it, is a mixture of blue ocean, white clouds, and green and brown land. Mars is covered with a dust which contains a lot of iron oxide (rust) which makes it orange looking. Jupiter is a a gas planet with an atmosphere that is mostly hydrogen, helium, and some bits of water, ice crystals, and other elements which make it white, orange, brown and red. Saturn is another gas planet similar to Jupiter but with different stuff in its atmosphere which makes it a yellowish brown color. Uranus is also a gas planet with a lot of methane which makes it a greenish blue color. And lastly, Neptune has a lot of methane gas which makes it bluish.

Different materials reflect different colors of light in space just as they do on Earth, and different planets are made of different materials. Why one planet is made of one thing while another planet is made of something else - ah, that is a good question, and one that I don't think we understand so well.

Planets have different colors because they are composed of different chemicals. Chemicals that are colored are that way because their electrons "relax" in ways that correspond to different wavelengths of visible light. Each chemical element or compound will have a unique electronic structure, so some will have different colors.