Why is it not plural form like "red lipsticks"?
Native speaker’s answer
Rebecca
The speaker refers to "red lipstick" generally, without referencing a specific type. This is why it is in the singular form and not the plural form. Lipstick is usually referred to in the singular form unless the sentence specifically states that there is a large quantity of lipstick. Ex: Red lipstick suits a variety of skin tones. Ex: I have over twenty lipsticks.