The titanium grid fins are also larger, which gives them better control authority. The center booster (like regular Falcon 9s) carries the interstage. This is light weight, but has significant aerodynamic effect when re-entering engines first for landing, like the feathers on an arrow. The side boosters lack this - the nose coses are much smaller - so are less stable aerodynamically. The likely reason for using the titanium fins is to give better control. They don't need the extra heat resistance for the side boosters - the center booster will have a toastier reentry as it's going much faster when it shuts down.

No, the ablative material is white (like on the center core). The darker fins are the fancy new titanium fins (note they're a little larger too).

Not sure why they went with the new fins on the side cores and not the center core... The side cores are going to have a pretty tame reentry (gentler than a Falcon 9 to LEO), while the center core will be coming in very hot. Currently they only use the titanium fins on very hot GTO reentries.