It's a short range force called the strong force.
As you may know, the proton is not an elementary particle, it is made of three quarks plus a sea of gluons. Gluons are the elementary particles that transmit the strong force. Particles that are sensitive to the strong force carry one of three colors.
The term color here refers to a quantum number that is more like a charge. it is only because there are three strong charges and three fundamental colors that by analogy, the strong charge is called the color. The strong force, unlike the electromagnetic force, is confining. That is a proton cannot be broken in free quarks. These are always bound.
The strong force, at the distances of the order of the proton radius, is 60 times stronger than the electromagnetic force see the wikipedia article on fundamental interactions