hmm..
One way is to create Tiles that fit perfectly together and build a transition between two ground/wall types. But thats much work to create all the middle tiles

another way would be, for example, to put a red and a yellow tile to be half overlapping each other.
And the shader now blends the pixels of the tile obove, to the tiles below.

But i always think that could be ugly, so in my early projects, i used my first method ^^"