I think they're single colour LEDs mixed in a matrix. Simplifies the circuitry and failure modes. I recall manufacturers had difficulty matching the standard light colours and, at least at the beginning, a mixture of fixed colour chips were used to produce the exact yellow and green colours required. There were also problems with temperature stability of the yellow in particular and early Dorman models had a clever system to vent power supply warmth across the array to keep it at a constant temperature. I made one of the earliest prototype LED boards for a railway signalling application in the early 1980s while a trainee in the Western Region technical investigations lab at Reading. It was a red level crossing boom lamp, possibly one of the worst environments imaginable for filament lamps with the frequent hard shocks of rising and falling. I remember it shining out of the office window on long term test for ages after I'd moved on to other things.
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