On UK airport charts like the one below for Manchester (EGCC), what is the difference between an identifier with white lettering on a red square and one with yellow lettering on a black square with a yellow outline?
Both are holding position signs. Both reflect the physical sign color the pilot would see, below is the red J2 and black J1 as seen from the cockpit (YouTube):
The red is for intermediate holding positions:
Where it is considered necessary to locate taxi-holding positions other than at runway/taxiway intersections – for example at taxiway/taxiway intersections in order to protect a priority route – the holding position should be identified by a single sign located wherever practicable on the left side of the taxiway. The sign consists of a combination of the letter designating the taxiway and a number identifying the hold position e.g. A1, A2, B2 etc. in white on a red background.