Hadn't noticed it, but just checked and yes the speed stays black on mine with SG/MY on-board. You could try setting it to different Locales to see if that enables it, sometimes different country settings enable/disable features, e.g. in 765 Singapore will enable travel guide but you lose exit icon in 'Where To?' menu. In my nuvi 3597 setting locale as UK or Luxembourg etc will enable an audible over-speed alert which isn't available for US or AU for instance. Have a play around, numerous hard-reset won't hurt it.
It's often a 'tradeoff' for features when you cross-flash firmware. "Garmin giveth and Garmin taketh away".