SUNN denotes the number of colors. Trick[SUNDelta[a, a]] yields nc2−1n_c^2 -1nc2−1.
SUNN
Trick[SUNDelta[a, a]]
Overview, SUNSimplify, Trick, SUNIndex, CA, CF.
SUNSimplify[SUNDelta[SUNIndex[a], SUNIndex[a]], SUNNToCACF -> False]
N2−1N^2-1N2−1