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Well unfortunately I haven't worked yet with low level computer vision using OpenCV (I made that with MATLAB) so I can´t give you specific advice on functions, I have here a link talking about the morphological operations: LINK

But let's explain a little more:

In order to use the morphological (or blob) operation we need to convert the image you have (in color) to black and white (not gray scale) in order to do this you should choose a threshold (it could be more than one depending of the needs).

The point is that after analyzing the histogram you could search for a "mode" corresponding to the yellow rectangle (or even the numbers if you take only the higher part of the image). Check this: LINK

Then you could make use of the blob operations.

Hope it helps.