UI Colors can be managed from the UI Colors menu accessed via Hamburger Menu.
They can also be managed via any Node that uses UI Colors. Tap the color preview to select a new color, then select the plus to add a new color or the pencil to edit existing colors. Editing existing colors will change that color for any Node is using it.
Create new colors by tapping the Plus Icon in the upper right.
Tapping a color from the list gives you a few option for editing the color.
Tap the name to change the name of the color. Tap the Trashcan Icon to delete the color.
Use the Color Picker to change the color. The outer circle of the color wheel select the hue while the inner circle selects the lightness and saturation.
The slider below the Color Picker adjusts the alpha or transparency of the color.
The larger circle in the lower left displays the currently selected color. Tap this preview to bring up details about the selected color including it's HEX value (#99ff00 in the example above). You can tap the HEX value to manually edit it. This allows you to more precisely match colors.
The smaller circle in the lower left displays the last selected color. Tap this circle to turn the current color into the last color.
The last selected color will be saved when moving between different features that use colors. For example you could create a color in the UI Colors and then also use it as the Ambient color of a Visual Material.