Some artists have a real system for how they place their paint on the palette. I suspect it depends on how many pigments they prefer to work with – more pigments, more organization required. Some portrait artists in particular have very complex specific arrangement of flesh tone pigments and specific mixtures of these on the palette. Other artists like to use a colour wheel format. Since I prefer to work with fewer pigments, ideally 4 or 5 maximum, I don’t really fuss with where I put them on the palette, with one exception – the colour white. If I am using a white paint, I like to put it in the center of the palette so that I can easily pull it into the other colours that are placed around it. With white oil paint, I put a long ribbon of it in the middle of the palette. Then I can pull it into darker colours from one end, and lighter colours from the other end, and not worry too much about the white getting contaminated.