Apparently schematics are wrong, then:
void galaxian_state::frogger_draw_background(bitmap_rgb32 &bitmap, const rectangle &cliprect)
/* color split point verified on real machine */
/* hmmm, according to schematics it is at 128+8; which is right? */
background_draw_colorsplit(bitmap, cliprect, rgb_t(0,0,0x47), 128+8, 128-8);
Putting the split back at 128 make MAME match the PCB video.
Formerly (un)known as hanamichi on the mameworld.info message boards.