Mexico’s allure for many visitors begins at the beach where white sand and turquoise water meet. But inland, a different kind of magic unfolds in the country’s Spanish colonial cities.   One colonial beauty often overlooked is Morelia, the capital of Michoacán in north-central Mexico. Built largely of rose-colored Cantera stone, the city of about 750,000 is more visually unified than many colonial cities. That architectural cohesion isn’t boring; it gives the city a sense of refined elegance.   I arrived in Morelia on a small-group...