+
+/** Class of indices for color algebra objects, to tell them apart from
+ * other index families like Lorentz indices. The color indices are the
+ * indices of the SU(3) generators T_a, so they lie in the range of 0 to 7.
+ * Note that this kind of index is different from the indices carried by
+ * elements of a specific matrix representation of su(3). Such indices are
+ * always hidden in our implementation and we are not concerned with them. */