The appearance of the rammed earth can be modified with pigmentation. Here is a test with red iron oxide, black iron oxide and yellow ochre used in 4 different quantities.
The yellow ochre gave a very small difference in appearance compared to the others. This could be because yellow is closer to the colour of earth compared to red and black. The pigment giving the yellow colour to the ochre is in fact iron oxide. But I noticed that the iron oxide content in the yellow ochre is only 10-15 % compared to 90 % in the red and black iron oxide, so this could also be a reason for the small difference.
To test the pigmentation in a bigger rammed earth cube I mixed increasing amounts of red iron oxide with 2,5 kg of earth.
It could be interesting to allow for deeper erosion in some parts of the façade. This could be done by directing the water from the roof to certain areas on the façade instead of in pipes. If the façade is pigmented as a function of depth, then the colour would change depending of the depth of the erosion. To test this I rotated the red gradient rammed earth cube and applied high pressure water to it.
The eroded pigmented rammed earth turned out less red than the uneroded red surface. Since the pigment is mixed with the smaller particle sizes which wash away before the bigger stones. The washed stones therefore don’t have any pigment left on them so there is less red on the surface. It might be a better idea to use stones in different colours mixed in at difference depth to achieve the effect that the rammed earth is shifting in colour as the erosion depth increases. For example, using white/bright stones would be interesting because the building would turn white as it gets eroded and older, as opposed to a concrete façade which gets dirty and darker over time.
The pigment washed away even with shallow erosion, so pigmentation is probably more suitable indoors where there is no erosion. Since the rammed earth is compacted in separate layers it is easy to achieve gradients like in the photomontage below.