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No. But it really depends on the application technique and paint type. There is no question that color sanding and buffing the top coat (regardless of clear or color) to ensure a perfectly flat surface will give you the best appearing result (reflectivity), but most of the time the top coat is just buffed with a machine buffing compound. Color sanding takes a lot of time and if you're not doing it yourself, really punches up the paint shop bill.
And, for what it's worth, if you clear-coat your color you do not color sand between applications. The reflectivity of the finish is achieved in what is done to the top coat.
Required?...mmmm...Even the best get some orange peel or imperfections sometimes. Unless you are shooting for a factory 'orange peel' look, I think most will agree the only way to get that perfectly smooth, flat glass look is to smooth it out by wetsanding and buffing.
Application technique, paint type, conditions will determine 'how much' color sanding you might need to do.