Nope, there are two palettes for sprites which source from the 4-shade gamut. IIRC GBC uses 8 palettes for the sprites and those are sourced from that 5:5:5 RGB cube with the really weird gamma ramp and side effects.
Yeah, had a look around and there are indeed two palettes, but the only difference is the transparent color as far as I know. There's still a total of 4 colors only obviously.
