What happens if you use hair color without developer?

If you leave a colour cream without a developer in the open it will eventually turn dark due to oxidation from the air. But slowly. The ammonia in the colour cream opens up the cuticles and deposit the colour pigment. Without the developer, it cannot bond and develop into desired colour inside the hair shaft.

Do I need to add developer to color?

The developer helps open the hair cuticle and activate the hair color. If used by itself (ie without color or bleach) the developer will lift the hair color, but the color result will not be good. For best results, you need to mix developer with a color.