In a previous post yesterday, "Mixed colour components diffuse differently", I wondered why the grey tone of my clouds splitted in the two different basic colours again.

Thank you for the members who replied on the topic, but I also continued my investigations, as described here.

In this picture I wetted my paper with plenty of water and draw a small line of each basic blue and red colours I use in my palette.

Remark how LightRed an CadmiumRed diffuse a lot more than other colors.

French Ultramarine and Cobalt Blue diffuse almost the same.

These characteristics (specially from the red colours) will explain partially why my color mix components 'separated'.

In a next test I wetted my paper and drew every minute a small line with French Ultramarine, Cobalt blue, Light red and Cadmium red.

Observe the amount of diffusion in function of time.

The last test was done similar to the above test but with four different mixtures:

Row A: French ultramarine + Light red

Row B: French ultramarine + Cadmium red

Row C: cobalt blue + Light red

Row D: cobalt blue + Cadmium red

Conclusions:

• the separation is observed mainly when the paper is too wet (first 5 minute)

• also the type of blue and red components have impact on the splitting (because of their diffusion characteristics)

• the main reason for colours from a mix to split is the different diffusion rates of the component colours in a too wet environment.

Some random tests:

note that the direction of the separation can be anyway (upwards, left, right down), it does not seem to depent from gravity only.

I remember from a tutorial from Olly: "timing is critical in wet on wet painting". Beside type of paper and amount of water, I also learned that the colour(pigment) plays an important role.