SUNTrace[exp] is the head of color traces. By default the trace is not evaluated. The evaluation occurs only when the option SUNTraceEvaluate is set to True. It is recommended to use SUNSimplify, which will automatically evaluate all color traces involving 2 or 3 matrices in the input expression.
Overview, SUNSimplify, SUNT, SUNTF, SUNF, SUND, SUNTraceEvaluate.
SUNTrace[SUNT[a, b]]\text{tr}\left(T^a.T^b\right)
SUNTrace[SUNT[a, b], SUNTraceEvaluate -> True]\frac{\delta ^{ab}}{2}
SUNTrace[SUNT[a, b]] // SUNSimplify\frac{\delta ^{ab}}{2}
SUNTrace[SUNT[a, b, c]] // SUNSimplify\frac{d^{abc}}{4}+\frac{1}{4} i f^{abc}
SUNTrace[SUNT[a, b, c, d]] // SUNSimplify[#, SUNTraceEvaluate -> True, SUNIndexNames -> {j}] &\frac{1}{4} \delta ^{ad} \left(C_A-2 C_F\right) \delta ^{bc}-\frac{1}{4} \delta ^{ac} \left(C_A-2 C_F\right) \delta ^{bd}+\frac{1}{4} \delta ^{ab} \left(C_A-2 C_F\right) \delta ^{cd}-\frac{1}{8} i f^{adj} d^{bcj}+\frac{1}{8} i d^{adj} f^{bcj}+\frac{1}{8} d^{adj} d^{bcj}-\frac{1}{8} d^{bdj} d^{acj}+\frac{1}{8} d^{cdj} d^{abj}
SUNTrace[SUNT[a, b, c, a, b, c]] // SUNSimplify\frac{1}{4} \left(C_A^2+1\right) C_F \left(C_A-2 C_F\right)