Life of a star
Nuclear fusion provides the star with this force in the form of pressure that acts in the opposite direction of the gravity. As long as the inward force of gravity and the outward force generated by the fusion reactions are equal, the star remains stable. This phase is called the
main sequence.
Stars in the main sequence vary in their sizes, temperature and eventually colors. So, how hot the star is related to the color of light it emits.
Our sun is an example of the main sequence. Since the sun has relatively average size and temperature comparing to other stars. It emits a yellow type light. While, hotter stars emit a bluish light or a light in the Ultra Violet region. And cooler, smaller stars emit red light.
What makes a star hold itself up against the gravity?
Once a star is born, gravity starts to act upon it. Hence for a star to live, it should exert some sort of force against the gravitational collapse in order not to crash down.