You can simply use World#playEvent(int type, BlockPos pos, int data) to spawn a splash potion effect, where type is the event to play, pos is the position, and data is the case-specific data.
In this case, you want either type 2002 or 2007 to play the splash effect. 2002 has the particles of a normal potion, while 2007 has the particles from an instant-effect potion (instant damage, for example). With this event, the data corresponds to the color of the potion particles.