The logo needs to be either square, or a have a square element to it so that it can, for example, fit neatly in the corner of a page and be recognisable. The QPPI brand needs to be a 'stamp of approval' on documents and other media so that client's companies might feature the logo on their websites and brochures to promote their competency approval through the QPPI.
It needs to be clean and clear; work in greyscale and monochrome as well as colour.
I would suggest the initials QPPI with a graphic and then a 'removable' slogan beneath (which is simply the expanded initials).
In terms of colours, just two would be good, maximum three. Three shades have been chosen but there's no particular preference at this point.
Examples worth considering are: