Art instructors are asked, "What colors are on your palette?" or "How do you arrange your colors on your palette?" It is more important that the artist be able to find a color quickly without interrupting his thought process while painting. To do this, he must be consistent when he arranges his palette.
Some painters put all the cool colors on one side and the warm colors on the other side. Some put white, black, and the earth colors in a separate location from the others, across the top or bottom. Any of these arrangements is fine, but you will find that your painting process is smoother if a given color is always placed in the same location on the palette.
During the painting process, especially the mixing process, you are making many decisions in nanoseconds. As quickly as you can flick the brush, you decide which out-of-the-tube color you need and whether to pick up a large or small dab. Then, you move on to compare the mix with the color area on the subject. When this process hits a speed bump in the form of a search for the right color, you are likely to be stalled and think, "Now what was I mixing?" Such interruptions break the concentration artist and the tempo of paint mixing and application. Both are necessary for unity within the finished work.
Here is my preferred arrangement, which I place on the left side of the palette. (I am right-handed.) With white closest to me and black at the far end, I place the colors in the order of the color wheel including the earth colors where they fit. I vary the colors chosen by the subject matter, the number of tubes I want to carry on location, or by mere whim.
In order, left to right: Daniel Smith Mixed White, lemon yellow, cadmium yellow light, yellow ochre, cadmium orange, burnt sienna, cadmium red, thalo red rose (Grumbacher), alizarin crimson, magenta (Winsor-Newton), ultramarine blue, cerulean blue, thalo turquoise, thalo green, black. (I place a strip of saran under the colors for easier clean up.)