Why do I need different shades of green foliage for my trees?

Using several shades will add more realism to your layout and create depth.

This is how trees are in real life: when you are next going past some trees, stop and take a look at the different shades of green, even take some photographs for reference later on. This will give you a better idea of how things look in nature rather than trying to remember what you saw.

Also depending on what time of year you are trying to recreate, the colours of the foliage will change.

When adding your foliage to larger wooded areas add the different shades of foliage randomly to the canopy otherwise it may look too uniform and contrived.

The same principle should be applied when creating hedges. The results will speak for themselves.