Defining a SU(3) gauge group:

Automatic contraction with indices of fundamental representation:

Delta[a,a] returns the dimensions of the representation of a:

For contracting with Field and other objects besides Delta use Contract:

Automatic index contractions do not work with flavor indices:

For more details regarding the associated issues see the documentation of Contract (in particular the section on "Possible Issues").

For flavor indices Contract[Delta[p,p]] returns the number of flavors that p can assume, which can be symbolic: