I have configured HTB on a bonded interface and under certain conditions, in Winbox the queue change colour from Green to Red. No packets are dropped during this time. Stopping the traffic causes the queue to change colour to Green

Any idea on when a Queue is RED, what it means and how I can investigate further.

Green = 0 - 50% utilized
Yellow = 50 - 75% utilized
Red = 75 - 100% utilized.

and in general it has nothing to do with limiting and dropping - it just shows current usage of the queue =)