I want to understand the 'vertex factor' of 3-bosons field and 4-bosons field but get confused. (I know the lagrangian and have computed the interaction vertices already) only need to understand the vertex factor. In other words, I want to learn how 3-boson vertex and 4-boson vertex are computed in Feynman diagrams. It is, for example, given in Peskin&Schrioeder on page 507 but I dont get it as to what is permuted there, the color indices a,b,c or the lorentz indices and the momentum? any help/hint is much appricaited ! Thanks.