The color function for tetraquark configuration $(qq)(\bar{q}\bar{q})$ is shown following: $$ \zeta_1=\frac{1}{2\sqrt6}[(rb+br)(\bar{b}\bar{r}+\bar{r}\bar{b})+(gr+rg)(\bar{g}\bar{r}+\bar{r}\bar{g})+(gb+bg)(\bar{b}\bar{g}+\bar{g}\bar{b})+2(rr)(\bar{r}\bar{r})+2(bb)(\bar{b}\bar{b})+2(gg)(\bar{g}\bar{g})] $$
for $|6\bar{6}\rangle$ color $\text{SU}(3)$ representation.
However, for example, why is the term $(rb+br)\bar{r}\bar{r}$ impossible in $\zeta_{1}$?