Essentially, you play with the colors until you get a nude shiny effect on your sim's lips. However, you're not limited to plain colors. You can play with the channels to create sultry, shiny lipstick as well.
Also, to play up the "natural* theme, all of the in game shots (the first photo of the post) were taken in normal outside light. No outside lights were used to enhance the texture.
Anyhoos, Sunkist Lipgloss is available for females teen and older.