Density

All paint colours have a different density known as specific gravity.  This is determined by the pigments that are used to make the paint colours. Specific gravity is the weight of the pigment in water.  Some pigments, like Titanium White, are heavier than others such as Prussian Blue.  This difference in densities is exploited in fluid art to encourage cells to form.   

When preparing paints to pour, Titanium White is added to the cup first.  Other colours are added and the cup is then flipped onto the canvas.  This results in the heavier Titanium White, which is now at the top of the other paints, to sink through them which helps encourage the cells to form. 

Paint manufacturers produce charts showing the densities of the paints they make and it is a good idea to reference these when making your choices for your pours. I will add some links to paint charts in the next few days.