Don’t worry about clashing. It’s hard to go overboard with bold colors at a Mexican celebration, where decorations in hot pink, aqua, and lime meet their match in the bright flavors of the menu. Many party stores and websites (such as www.plumparty.com and www.pinatas.com) offer inexpensive Mexican-themed paper flowers, napkins, and piñatas. In keeping with the casual mood, you can forget about coordinating the glasses and plates and concentrate on what you put in them: an easy assortment of quick-cooking and store-bought dishes. As guests arrive, keep them happy with quesadillas, chips, and guacamole washed down with lemonade or sorbet margaritas.