Leonardo Da Vinci suggested using progressively bluer colours the further away objects are to create depth.
The tip is used in paintings although you can also use it for your Mind Map illustrations.
In the first of the examples above the darker blues appear further away even though the boxes are all the same size!
In the second of the blue images, adding perspective, the effect is even greater!
Subtle changes even to bold colours create depth.
Great tip from da Vinci 🙂