My notes say that once inside the yellow bar you are above Mcrit, therefore will experience some buffet. The black and red pole can be likened to the low speed one - it isn't a structural limitation (as in Vmo), rather a performance limitation. Mmo (the black and red) will activate the high speed clacker, indicating that you are not safe, and may experience a high speed shock stall.
The buffet margin is in respect of Mmo, the clacker, and possible shock stall. eg) if you pull 1.3g at the bottom of the yellow tape, you will get the clacker. Don't forget that the 0.3g marhin is only at the BOTTOM of the yellow bar and reduces to zero at the red and black. Therefore, half way up the margin is 0.15g eg) hardly anything! Any kind of turbulence and you will get some buffet.