first i'd tell the customer about their coloring (skin, hair), and also explain to them that blue based reds have a whitening effect on teeth while orange or warmer reds can yellow teeth- if they have a concern (careful on this as to not insinuate the customer has yellow teeth. I'd ask them if they want a red for a special occasion, or one that can be worn casually. I'd ask them then if they want a glossy/shiny finish or a flat/matte finish. Do they like a certain look? If they are totally clueless, I'd try a a couple "universal" reds on them and a couple that fit their coloring after sterilizing the product well. Once they have chosen I would try to sell a matching lipliner and tell the benefits and a lip primer as well. If they want more depth of color (can be tricky with red) suggest a lipglass.