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 hold position signs; white on red is "to protect a priority route".
— UK AIP GEN 2.3
Below is the red J2 and black J1 as seen from the cockpit (YouTube):
The usage description below shows inconsistency with the map, which hints at a gradual phasing out (see 2021 update below):
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. An example of an intermediate taxi-holding position sign is shown at figure 7.13 (F).
At Manchester, only yellow on black is used now. Latest AIP and CAP 168 still have the white on red option.