dither() leaves a colour that is on one of its steps flat and puts its pattern on everything in between, which is what a gradient is. A screen effect only sees pixels, so it cannot tell a tile meant to be flat from a sky meant to be dithered.
consoleColor(colour, POST_EFFECTS.COLOR_LEVELS.genesis) puts the colour on the steps where it is chosen: the tiles come out flat, and the sky still gets the pattern. The fewer steps a console had, the further the flat colour lands from the one asked for. POST_EFFECTS.COLOR_LEVELS has the Master System, Mega Drive, PC Engine, Game Gear, SNES, PlayStation, Saturn, Nintendo 64, Dreamcast and GameCube.
Click the preview first so it hears the keyboard. C changes console, from a Master System's 4 steps to a GameCube's 64. D switches the dither off and on, to see both rows look alike underneath.