The magenta is part of color-coding (CYAN / WHITE / YELLOW / MAGENTA), nothing to do with managed. The thumb rule is nice, but coincidental.
Are you sure? Just to give one example, (OK, Airbus are calling it blue here, but if you look at the display they actually use cyan - their blue is a much darker shade):
FCOM DSC-31-40: Airspeed
(3)Target Airspeed (magenta or blue)
This symbol gives the target airspeed or the airspeed corresponding to the target Mach number.
The target airspeed is the airspeed computed by FMGC in managed speed mode (magenta) or entered manually on the FCU for selected speed mode (blue).
My underlining.