here are the steps to achieve the square cement blocks.
1: grab ur cement block
2: recolor the black outlines of each 8x8 to somewhere else in the palette. remember, these cannot overlap into a color already used in the block. for example, i did this
(this link will break in time coz i used puush lol)
3: then, change these colors to match the ones used in the initial block. this creates the illusion that they use the same color without them taking up the same palette spot.
4: this tile
effectively functions as a corner line guide.
5: just merge it on-top of the cement block and voila
however, opening up mario world's level goes to show, well:
so there you go