I tried to change it, but couldn't get it to work. I tried overriding fields to match how hazard concrete is specified, and tried increasing transition_overlay_layer_offset on refined-concrete. It looked a little different, but still had the boundaries.
The colored concrete tiles originally come from the base game. Vanilla doesn't actually have items to place them, but the colored tiles already exist for whatever reason. So their appearance and their transitions aren't specified by Nullius. Nullius could potentially change their appearance, but so could a third party.
Nullius adds the white and orange tiles only. The colored tiles use the regular grey reinforced concrete and add a tint, but tints can't add color, only subtract it. Since the base is starting out as grey, you can't tint it to be white. And if you tint it to be orange it just ends up looking brown. So adding colors brighter than the regular grey concrete required a new base texture. It might make sense to switch yellow to that base texture too, but I think it looks OK as is. Theoretically, all of them including the original grey could be implemented by tinting the new brighter texture, but I haven't tested it to see how it looks. Probably the grey wouldn't look as nice, but some of the other colors might look better.
If you can figure out how to make the transitions blend together and still look decent, then go ahead. You can submit a patch to the git hub, which is linked from this page. If I think it looks acceptable, I'll include it in the next update. I don't object in theory.
