In this MOC I use DOTS to simulate two coloured liquids being poured onto a surface with mixing between them at the interface.

The code starts with a rectangle and covers half of it in squares of one colour, and half in squares of a second colour. Then row by row, tiles are flipped colour based on a probability. The closer to the dividing line between the colours, the higher the probability of a square changing colour.

Finally each tile is assessed in turn based on the surrounding colours and the shape changed to be a pizza tile or half-circle if appropriate to try and create that flowing look. If the baseplate were the same colour as the lower colour, I might keep all that colour as squares and try and arrange the tiles in the top colour to make it like it was flowing down the wall.

The python code is available on GitHub, and as well as the 36 designs in the instruction PDF there are a bunch of examples as LDraw files for different sizes of rectangle with varying mixing parameters.