In string theory(s), are quarks just individual strings, or are they made of multiple strings? Are the heavier quarks made of heavier or longer strings? Are there red, blue, and green strings corresponding to the color charges of qcd?
OK, here is a simple answer. Almost all the data that research in particle physics has produced the past 70 years or so is described well by the theoretical Standard Model.
In the SM these are the elementary blocks that build up matter, and in the SM they are treated as point particles, dimension 0 in space. String theories have expanded the representation of particles from dimension 0 to dimension 1.
Each of those particles in the table above is represented by as a string in string theory. The quantum numbers that identify each particle in the table above are vibrational modes of the string. Each particle is a different string vibrating at its own "frequencies" , in the plural because the dimensions of spacetime are extended to ten or eleven and each particle is characterized by many quantum numbers, each of which should map into a "frequency".
There is no Standard String Model yet. The group structure that characterizes the SM exists in many forms of string models and the SM can be embedded in them, but a definitive model has not emerged yet. The hope is that eventually the SM will be embedded in the final string theory model which will incorporate the quantization of gravity and we will have a Theory of Everything (TOE).
The table above is simplified not showing the colors for the quarks. Color is another quantum number and it characterizes the frequency modes describing quarks as its other quantum numbers. So color is also a specific vibration of the string representing a quark. There are no "colored" strings otherwise.
String theory is a possible way to unify gravity and quantum mechanics. But strings were originally proposed as a way to describe the strong force that governs quarks i.e. quantum chromodynamics when the particles are at low energies.The strings are the tubes of glue that connect and confine the two quarks in a meson.
Effective string descriptions have also been considered for spin systems as the Ising model in 3D , where 2D surfaces separate the domains of “up” and “down” spin and hence computations involving two-dimensional worldsheets similar to the worldlines of a single particle—in other words a string rather than point particle picture seem natural.
Reference: Glimmers of a connection between string theory and atomic physics By Shamit Kachru, Physics 1, 10 (2008)