Greyscale output is useful for styling in a third party program like QGIS.
You can still choose the Greyscale colour schema (code number 9) as an output option which maps a received power value in dBm to a RGB value so -77 is RGB(77,77,77) and -88 is RGB(88,88,88). This is good if you need the precise value at a given point, especially when exporting as SHP.
You can also create a greyscale schema using your own levels and the colour palette tool. With this you can have buckets at fixed intervals eg. 10dB and only a few buckets.