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Yes. The prefix "cis" stands for "within" and "trans" means "outside". So, "cis-Neptunian" means "inside Neptune's orbit".
Because it spends most of its orbital period
farther from the sun than Neptune is.
Pluto is a dwarf planet and trans-neptunian object. Crossing orbits with Neptune stops Pluto being a planet.
Yes.
If you mean a large trans-Neptunian object: dwarf planet 136199 Eris then you have made a mistake. The dwarf planet Eris is thought to be quite white.