When the app is set to 50% of the screen (split with another app), the colors picker (and size picker) become one tool and are inconvenient to use - because changing the color makes it your primary, instead of just swapping the one in use.

There is actually quite a lot of space here when there are few tools used - so this split-screen view could work the same it works on the full screen - just show the few colors that fit the available space. The more tools someone use, the less colors he will see. This works on a 2/3 ratio of the split screen, so I am not sure why it doesn't on a 1/2 ratio.