Multicolour LED string lights work indoors and outdoors alike. They are usually not very bright and instead provide a faint glow. When the faint glow of one light combines with several other ones in many colours, it will start to look surreal. But you cannot use a multi-colour light that is meant only for indoor use for outdoor purposes. The difference is in the level and quality of the insulation on the wiring. The insulation for outdoor lights will be thicker and weatherproof. The plastic case covering the bulbs should be waterproof and dustproof as well.