Color of gluon mediating quark-antiquark process?

I have the following process:
$q(R) + \bar{q}(\bar{B}) \rightarrow q(R)+\bar{q}(\bar{B})$

In words: a quark with red color-charge and an antiquark with an anti-blue color-charge are incoming, and a red quark and anti-blue antiquark are emerging. Since I am not sure how else to draw that, I try to do some ASCII art here:
Code:
q(R)          q(R)
1\           3/
\          /
\        /
\      /
~
~ G (?,?)
~
~
/   \
/     \
/       \
2/        4\
q-(B-)      q-(B-)

----------------------> t
q are the quarks, q- the antiquarks, R is red, B- is antiblue, time runs from left to right.

The question is: which gluon(s) can participate in this exchange?

I have read Griffiths "Introduction to Elementary Particle Physics" on that topic and on p.290 Example 8.1 he more or less states exactly that problem. He talks about a "typical octet state" $R\bar{B}$. What is that supposed to mean? I thought the octet states are always a superposition of two states, in this case:
$$|1\rangle = (R\bar{B} + B\bar{R})/\sqrt{2}$$

I was thinking along the lines of color conservation. The only thing that made sense to me is that the mediating gluon has to be $B\bar{B}$: the incoming red quark sends out a $B\bar{B}$ pair. The incoming anti-blue antiquark combines with the blue charge, and what remains is an anti-blue charge... But I somewhat feel that this logic is flawed. :(