The BCC Color Match filter was designed to match the color and luma values from a source clip (a) and a target clip (b). It solves the common problem where abutting clips of the same scene that were shot at different times of the day but need to look like they were shot within seconds of each other.
The filter provides 3 point sample controls, from which the user samples regions from the highlight, midtone and shadow areas of the source and target (destination) clips. You start by applying the filter to the target clip and then disabling the filter so that it has no effect on the image. Then set the h/m/s samples for the source and target. Then enable the filter.
A bias control was provided for the h-m and m-s regions, effectively enabling the user to instruct the filter on what values are to be considered highlight and shadow from the mid-tones of the image. To use this, set the appropriate view (h/m/s etc) and change the value in the h-m or m-s bias parameters, switching the view until you are satisfied with the region settings. Then set the view switch back to normal.
At this stage you should be seeing that the source and target color and luma values are fairly closely matched.
Finally, adjust the master level and color parameters to back-off or increase the master color and luma matching. If needed, this can be further tweaked using the individual h/m/s level and color parameter sliders.
As with any color matching tool, it takes practice with the tool to really get comfortable with the controls. Once you use it a few times, I'm sure you'll really start to see the benefit of having this in your toolbox.