Why is Neptune blue?

Answer: Methane absorbs red light

Distance from sun — Wrong. How far a planet sits from the sun doesn't decide what color it looks. Neptune is blue because methane in its atmosphere absorbs red light and lets blue light bounce back out.

Frozen nitrogen on surface — Wrong. Neptune is a gas and ice giant, so it has no solid surface where frozen nitrogen could sit. The blue comes from methane in the atmosphere absorbing red light.

Methane absorbs red light ✓ — Correct! Neptune's atmosphere holds methane, and methane soaks up red and infrared light while letting blue and green pass through. That leaves the planet looking blue. Uranus has methane too, but it looks paler blue-green, and Neptune's deeper blue may come from something else we haven't pinned down yet.