Which interaction tends to hold the particles in an atomic nucleus together?
That would be the strong nuclear force. It is called the strong
force because it is the most powerful of the 4 fundamental forces;
typical field strength is 100 times the strength of the
electromagnetic force, 1013 times as great as that of the weak
force, and about 1038 times that of gravitation, according to
Wikipedia. It is also sometimes called the color force, however
this has no relation to the visible color we commonly refer to.